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How Does Light Travel?

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century’s BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern era, the 20th century led to breakthroughs that showed us that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein’s Theory of Relativity . However, there remains many unanswered questions about light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light to the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle’s theory of light, which viewed it as being a disturbance in the air (one of his four “elements” that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or “corpuscles”). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton .

The first edition of Newton's Opticks: or, a treatise of the reflexions, refractions, inflexions and colours of light (1704). Credit: Public Domain.

Newton’s corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise “ Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light “. According to Newton, the principles of light could be summed as follows:

  • Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
  • These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to “wave theory”, which had been advocated by 17th century Dutch astronomer Christiaan Huygens . . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his “ Traité de la lumière “ (“ Treatise on Light “). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment , which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young’s version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation, would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter’s moon Io to show that light travels at a finite speed (rather than instantaneously).

Prof. Albert Einstein uses the blackboard as he delivers the 11th Josiah Willard Gibbs lecture at the meeting of the American Association for the Advancement of Science in the auditorium of the Carnegie Institue of Technology Little Theater at Pittsburgh, Pa., on Dec. 28, 1934. Using three symbols, for matter, energy and the speed of light respectively, Einstein offers additional proof of a theorem propounded by him in 1905 that matter and energy are the same thing in different forms. (AP Photo)

By the late 19th century, James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell’s equations ) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c ).

In 1905, Albert Einstein published “ On the Electrodynamics of Moving Bodies ”, in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity , which reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect , Einstein based his idea on Planck’s earlier work with “black bodies” – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein’s photoelectric effect was attempt to explain the “black body problem”, in which a black body emits electromagnetic radiation due to the object’s heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes ).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein’s explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named “photons”. For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg’s “uncertainty principle” (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger’s paradox that claimed that all particles have a “wave function”.

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function , a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly “collapse”, or rather “decohere”, to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the “ Schrödinger Cat” paradox).

In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect. The animation below shows the delay scientists had expected to observe. Credit: NASA/Sonoma State University/Aurore Simonnet

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation ). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly divert it, or arrest it, is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

We have written many articles about light here at Universe Today. For example, here’s How Fast is the Speed of Light? , How Far is a Light Year? , What is Einstein’s Theory of Relativity?

If you’d like more info on light, check out these articles from The Physics Hypertextbook and NASA’s Mission Science page.

We’ve also recorded an entire episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel .

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56 Replies to “How Does Light Travel?”

“HOW DOES LIGHT TRAVEL?”

it travels lightly. 😀

Light doesn’t exist. This is an observation from light’s point of view and not ours. Traveling at the speed of (wait for it) light, absolutely no time passes between leaving it’s source and reaching it’s destination for the photon. This means, to the photon hitting your retina, it is also still on that star you are observing 10 light years away. How is this possible? Maybe John Wheeler was right when he told Richard Feynman that there is only one electron in the universe and it travels forward in time as an electron, then back in time as a positron and every electron we see is the same electron.

MY QUESTION IS: Whether light is a wave , particle or both.. where does it get the energy to move through space/time. In other words is the energy of light infinite? Does it continue on without lose of energy…..forever…….

I believe that Special Relativity says that the energy of light is infinite due to the very fact it has no mass. E=MC^2

In reverse, this is also why something with mass to begin with. If accelerated toward the speed of light, will see their mass and gravity increase to infinite points as they near relativistic speed (it actually starts around 95% with a steep upward curve from there), with a relative slowing to a stop of time.

Join the discussion

Light and the universe are only illusions that are formed in our minds via technology that sends information from the simulation program we’re living in. That information comes in the form of invisible wavelengths that includes wavelengths that we perceive as light. The visible retinas in our eyes are like tiny video screens where these particles are arranged into patterns that form into all the various objects we think are real objects. This information is also converted into thoughts within our minds which are like computer processors that process that information.

We are living in a computer simulation that is much more advanced than anything the characters in the program have built according to the information called the Beast.

Brad,…So You’re suggesting that “life” as we know and call it “is some kind of retro-virus” or “bio-intelligent format” heaped upon a perceived “set of accepted data sets” that are not in sync with each other in most cases with exception to Math 94% of the time….Even then it can vary which suggests Your idea would mean we all live in a fairy tale. That is what you suggest,…right?……

Brad has watched the Matrix too many times.

Correction: Even gravity doesn’t slow light down. Light (EM radiation of any wavelength) always travels at speed c, relative to any local inertial (Lorentz) frame. It could also be noted that the wavelength of an EM wave is not a characteristic of that wave alone; it also depends on the state of motion of the observer. You might even say, “One man’s radio wave is another man’s gamma ray.”

Light actually “slows down” every time it has to travel through anything but a vacuum. Look up Cherenkov radiation to see what happens when light initially travels faster than it can through a particular substance, like water. Light speed is not constant when traveling through any medium except pure vacuum. In fact that is why your pencil looks bent when you drop it in a glass of water. Light bends to find it’s fastest path through any medium, and it slows down in that medium.

if all you scientist could ever get it in your pie brain that there is no time, no light speed, no warping space, no black holes for the purpose of moving through space quickly, no smallest no biggest when it comes to space and that all of everything has always been in existence but not necessarily as it is now. you will never find the smallest because if it exist it has an inside, and you will never find the end of space because it is infinite.

What are you smoking?

The article started out nicely, but I lost interest as mistakes began to appear. First Einstein did not “propose” the photoelectric effect. The photoelectric effect was first observed by Heinrich Hertz in 1887. Einstein used the idea of photons to explain the photoelectric effect and derive the photoelectric equation. Also, Max Plank had already derived the blackbody distribution, by assuming that electromagnetic energy of frequency f could only be emitted in multiples of energy E=hf, by 1900. Einstein’s paper on the photoelectric effect was published in his “miracle” year of 1905. The photoelectric effect has nothing to do with black body radiation.

Einstein did not coin the name “photons” for light quanta, as stated in this article. This term was first used by Arthur Compton in 1928.

I have to say that I do not know what the author of the article means when he says ” calculating the wavelength at which light functioned” in reference to Louis-Victor de Broglie. Louis de Broglie used the dual nature of light to suggest that electrons, previously thought of as particles, also had wave characteristics and used this notion to explain the Bohr orbits in the hydrogen atom.

I gave up on the article after seeing these errors. I’m afraid I have a low tolerance for sloppy writing.

Oh, it’s BCE now, “Before the Common Era” BC has worked for 2000 years but now the PC police have stepped in so as not to offend who? Some Muslims?

mecheng1, you must be very young. BCE has been in used in academia for decades. It’s nothing “new”, just out of your circle of knowledge.

Decades??? Really?? How does that compare to 2000 years?

Only in Euro-centric texts have your assertions been true, McCowen. The rest of the world not influenced by Christianity have used their own calendars and a “0” year or a “year 1” from which to reckon the passage of time, largely based on their own religions or celestial observations.

Over the last century or so, through commerce, most of the world has generally accepted the use of a Western calendar (or use it along with their own for domestic purposes, like we here in the US still use Imperial units of measure that have to be converted to metric for international commerce). So, we are in a “common era” insofar as non-Christian societies are incorporating the Gregorian Calendar and the generally-accepted “year 1” established by that calendar (which is supposed to be the year of Jesus’s birth, but it probably isn’t according to current scholarship). Besides, the Gregorian calendar is an improved derivative of the Roman calendar – even the names of the months come from the Romans.

In short, it is more accurate, as well as respectful, to go with BCE in these global times.

Where is the information carried on a photon hitting my eye(s), or cluster/group/pack of photons hitting my eyes(s), that I see as other distant galaxies and planets going around stars?

That’s the mystery, isn’t it? Even in scattering, light remains coherent enough to convey an enormous amount of information.

Since the miniscule equal masses with opposite charges, that make up the photon structure, interact at 90 degrees, this induces a spin (a finding from the 80’s by the LANL plasma physics program) which creates a centrifugal force that counterbalances the charge attraction of the opposite charges. This establishes a stable structure for energies less than 1.0216 MeV, the pair-formation threshold, separating these “neutrino” sub-components by a specific distance providing wavelengths varying with photon energy. This composite photon propagates transversely at c/n, the speed of light divided by the index of refraction of the material traversed. In spite of the mass being defined as zero, for convenience in calculating atomic masses, there is actually an infinitesimal but non-zero mass for the photon that is required for calculations that describe its properties.

Tim, you poor guy! You have a discombobulated brain! Everything you wrote is just gibberish.

i would like to know the temperature in a black hole…maybe absolute zero? is absolute zero the moment that time stop?

I think the temp inside a black hole would be extremely high since temperature seems to increase with mass. Comparing absolute zero to time stopping is very interesting though. To the observer they would appear the same.

Theoretically there is no temperature in a black hole from any observer POV because time is stopped. Although JALNIN does bring up that point, and he also brings up the point of increasing mass corresponding to increasing energy. Everything in Hawking and Einstein’s equations though, suggest that any energy would be absorbed back by the singularity, so there wouldn’t be any heat. In fact it should be infinitely cold. But time is no more, so technically no heat or energy is emitted anyway from any observers POV. Yet recent images of black holes from Chandra show that they emit powerful Gamma Jets along their spin axis just like Neutron stars, and Pulsars. BTW edison. The accretion disk can reach temperatures of 20MN Kelvin on a feeding SM black hole (quasar). NASA just published an article on it through the Chandra feed a while back.

Light doesn’t travel, it just IS. It is we, the condensed matter, that travels, through time.

Oh really? Is this just your imagination/illusion or you have published a paper on it?

So you don’t believe you travel through time?

I wish I understood just a portion of I just read, love sicence so bad BUT, sighs

It would be easier to understand if it wasn’t pure gibberish written by someone with no science background.

I have two “mind-bending relativity side effects” to share. At least they are mind-bending to me.

1) Light travels the same speed relative to all particles of mass, regardless of how those particles move relative to each other:

I can conceptualize this if we are only talking about two mass-particles/observers and the examples I’ve seen always involve only two observers. But if you have many mass-particles/observers, how does the space-time seem to know to adjust differently for all of them. I am sure i am understanding this correctly as it is a basic concept of special relativity and nobody seems to bring this issue up. But it “bends my mind” when i try to include more than two observers. Maybe you can help.

2) General Relativity’s (“GR”) prediction that the big bang started with “Infinite” energy and now the universe appears to have finite mass energy and Regarding the first effect: How can something infinite turn into something finite? Is the answer that at that early in the universe, quantum takes over and GR’s prediction of infinite mass-energy at the start of the universe is just wrong?

I need to correct a typo in my previous comment. Where i say “i am sure am understanding this correctly” I meant to include the word NOT. so it should read “i am sure am NOT understanding this correctly” Mark L.

Mark,….I think you’re understanding it just fine from the standpoint of multiple observers, The point might be that in space, the density of “emptiness” or “lack of emptiness” might be impacted from one area of observation to another by an observer who’s perceptions are not equal but not being taken into consideration by each observer. ( an example if I may?) If you were to use a Clear medium which is oil based beginning with 5 gallons of mineral spirits in a large barrel and keep adding 5 gallons of thicker clear oil and then heavy grease and stop with using a clear heavy wax,…what happens is you end up with a barrel of clear fluid that begins with a floating substrate but the liquid begins to keep floating and the heaviest stuff goes to the bottom,…You end up with a sort of solid tube of clear fluids which if you could keep them in shape here on the earth, “you could observe them” from several positions, #1. the fluid end #2, the less fluid part, #3, the semi solid part #4. the seemingly solid part #5. the almost solid part & #6. the solid part……all of which would be transparent….You could then shine a laser through all of it and perhaps do that again from different places and see what happens at different angles…..I think what happens as a result would be, an observer would end up be influenced as per his or her ideas thusly because of the quasi-nature of what the density of space is at the point of space is where the observation is made. just a guess.

All Special Relativity really says about light is that it appears to move at the same rate from any observer POV. There are other more advanced rules relating to light speeds. One of them is the implication of infinite energy in a photon because of the fact it’s mass-less, therefore it can move at the maximum rate a mass-less particle or wave can (not necessarily that it does) Later when the electron was discovered (also mass-less particle or wave), it was also found to conform to the rules of special relativity.

As far as the big bang, there are a lot of cracks in that theory, and many different ones are beginning to dispute some of the common ideas behind the “Big Bang” as well as “Inflationary Cosmology”. Honestly though, both standard and quantum physics applied, and yet both went out the window at the same time at some point. That’s what all the theories really say. At some point, everything we know or think we know was bunk, because the math just breaks down, and doesn’t work right anymore.

i think until there is an understanding of the actual “fabric” of space itself, the wave vs particle confusion will continue. another interesting article recently was the half integer values of rotating light. planck’s constant was broken? gravity? a bump in the data? lol these are interesting times.

There’s no fabric.

Tesla insists there is an aether, Einstein says not. Tesla enjoyed far less trial and error than Einstein. The vast majority of Tesla’s projects worked the first time around and required no development or experimentation. I’ll go with Tesla; there is an aether as a fabric of space.

http://weinsteinsletter.weebly.com/aether.html

Maybe Special Relativity is not correct? 🙂

Feynman said unequivocally that QED is NOT a wave theory. In fact, the math only looks like Maxwell’s wave function when you are looking at a single particle at a time, but the analogy breaks down as soon as you start looking at the interactions of more than one, which is the real case. There’s no light acting alone, but always an interaction between a photon and some other particle, an electron, another photon, or whatever. He said “light is particles.” So the question re: how can light travel through a vacuum if it’s waves is a nonsensical question. There are no collapsing wave functions in light. There’s only probabilities of position that look like waves on a freaking piece of paper. Even calling light properties as “wavelengths” is nonsensical. Light comes in frequencies, i.e., the number of particles traveling tightly together. Higher frequency is more energy because it’s more particles (E=MC[squared]). “Wavicles” is pure bullshit.

I don’t agree with the John Wheeler theory that there is only one electron since the computer I am using was built by ion implantation and uses a very large number of them simultaneously to function.

Black holes don’t stop or slow light, if they even exist. A black hole could phase shift light, which is why we see things emitting xrays and call them black holes….but they could be something else too.

Photons have no mass but they do have energy. Energy and mass are transformable into each other. Gravity works on energy as well as mass. As massive particles approach the speed of light their measurable mass increases to infinity. But since energy is equivalent to mass, why doesn’t the photon, which has energy, not seem to have infinite mass?

NO other wave travels thru a vacuum? what about radio?

Radio waves are a specific frequency range of light.

Technically speaking, radio waves are emitted at various frequencies that share the same space time as light. They are not however light. They’re modulated electrons. Modulated photons certainly can be used to carry a vast amount of information a great distance. It cannot do it any faster or better than a radio wave though. Both electrons and photons are mass-less, therefore they both conform to the rules of Special Relativity in the same way. Both travel at the speed of light.

I just don’t understand is it a particle of a wave? It seems like it behaves like wave and sometimes like particle and in some situations is like a what ever you are going to call it.

So, the logical idea would to have formula Photon_influence * weight_for_particle + Wave_influence * weight_for_wave

Make it more compact.

This article is good but the title is bad as by the end we still weren’t told how light travels through space. Also, there are some historical mistakes as already pointed out. Now for my contribution: I think that light and Gravity have a lot in common; for one – an atom’s electrons transmit light and an atom contains the tiny heavy place that knows everything there is to know about gravity, that is, the nucleus. Light and Gravity are both related to the same entity, the atom. Unfortunately, we, still cannot grasp how what’s heavy brings about gravitation. For those of you with a creed for new ideas go to: https://www.academia.edu/10785615/Gravity_is_emergent It’s a hypothesis…

Gravity and light are infinite, like space and time… Mind the concept that there are waves within waves, motions within motion, vibrations within vibration, endless overtones and universal harmony…

From this article, I have “And in the end, the only thing that can truly slow down or arrest the speed of light is gravity”

Doesn’t light slow down in water and glass and other mediums. I was only a Physics minor, but I do remember coivering this though way back in the early 80’s. And in my quick checking online, I found the following.

“Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index = 1.3; see Figure 1) and 200,000 kilometers per second in glass (refractive index of 1.5).”

Were they saying something else here. I did like the article.

Photons are not massless, but their mass is incredibly small even compared to a proton or neutron. So, by Einstein’s E=MC^2, the energy required for a photon to move is greatly reduced, but photons do have mass and are affected by gravity. If photons had no mass at all, then gravity would have no affect on them, but gravity does. Gravity bends light and can change it’s course through space. We see that in the actual test first performed to prove Einstein’s theory buy observing the distorted placement of stars as their light passes near the sun observed during an eclipse. We can also see it through gravitational lensing when viewing deeps space objects. And the fact that there are black holes that are black because light cannot escape it’s gravity. So photons do have mass, be it miniscule, and with that their propagation with light waves through space will eventually run out of energy and stop. but this would probably require distances greater to several widths of our universe to accomplish. Light from the furthest reaches of the universe are not as bright, or as energetic, as they are at anyplace between here and their origins. That reduction in their energy is also attributed to Einstein’s equation and the inverse square law, where the intensity of light is in relation to the inverse square of the distance. That proves that light looses energy the further it travels, but it still moves at the speed of light. As light looses energy, it doesn’t slow the light wave.

It has been proven that more energetic light does in fact travel slightly faster. You can find the experiments done with light that has traveled billions of light years, the more energetic is in fact faster over a number of seconds, around 10 -15 or so. As people encounter this information, they see that many accepted theories can now be debunked.

The point of the article is nothing new; light acts like a particle AND a beam. So when you sit behind a closed door and someone shines a light on the door, the light will engulf the door and wave through and around the edges, the particle does not just bounce straight back. You can focus a beam of light on an object, but it will sneak though the corners and underneath the door, through any opening,. And yes, light travels forever. It is a constant, that cannot be sped up. We can slow it down by focusing it through prisims or crystals. But it still is traveling at 186,000/MPS.and that speed does not change. So, that is why we can see the outer edge of the universe: 13,8B light years away *the time that it takes for light to travel in one year, is one light year. So, it has taken 13,8B light years for the light of other galaxies to get here, so those galaxies could be gone by now, since it took so long to reach us, We are truly looking back in time as we see the light emitted from those galaxies and stars.

It propagates through the quantum mish-mash know as the aether . . .

If light is a particle and particles have mass why does not the mas increase with it speed?

Wow…there are errors in the article, yes…the enthusiasm demonstrated by all the comments is encouraging…but when I read these comments, I am a bit dismayed at the lack of understanding that is evident in most of them…confusing energy and intensity and wavelength…confusing rest mass and inertial mass…not to mention some off-the-wall hypotheses with no experimental evidence to support them. There are some great primers out there…books, documentaries, podcasts (like Astronomy Cast). Good luck!

Precisely correct. Sci-fi rules basic physics, which reflects on the poor education system. Pity.

First time I heard about A. A. and his theory about light I really didn’t like him. Why? Because light was the the fastest thing in the universe and there is no other thing faster than the light. Later, when I have red about angular speed I have asked my self if you have linear and angular speed and both of them are speeds how that will result in the maximum speed. Since then, I have not had a chance to get right answer.

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  • Sound & Light (Physics): How are They Different?

How Does Light Travel?

Light bends at the interface of two media.

Sound & Light (Physics): How are They Different?

The question of how light travels through space is one of the perennial mysteries of physics. In modern explanations, it is a wave phenomenon that doesn't need a medium through which to propagate. According to quantum theory, it also behaves as a collection of particles under certain circumstances. For most macroscopic purposes, though, its behavior can be described by treating it as a wave and applying the principles of wave mechanics to describe its motion.

Electromagnetic Vibrations

In the mid 1800s, Scottish physicist James Clerk Maxwell established that light is a form of electromagnetic energy that travels in waves. The question of how it manages to do so in the absence of a medium is explained by the nature of electromagnetic vibrations. When a charged particle vibrates, it produces an electrical vibration that automatically induces a magnetic one -- physicists often visualize these vibrations occurring in perpendicular planes. The paired oscillations propagate outward from the source; no medium, except for the electromagnetic field that permeates the universe, is required to conduct them.

A Ray of Light

When an electromagnetic source generates light, the light travels outward as a series of concentric spheres spaced in accordance with the vibration of the source. Light always takes the shortest path between a source and destination. A line drawn from the source to the destination, perpendicular to the wave-fronts, is called a ray. Far from the source, spherical wave fronts degenerate into a series of parallel lines moving in the direction of the ray. Their spacing defines the wavelength of the light, and the number of such lines that pass a given point in a given unit of time defines the frequency.

The Speed of Light

The frequency with which a light source vibrates determines the frequency -- and wavelength -- of the resultant radiation. This directly affects the energy of the wave packet -- or burst of waves moving as a unit -- according to a relationship established by physicist Max Planck in the early 1900s. If the light is visible, the frequency of vibration determines color. The speed of light is unaffected by vibrational frequency, however. In a vacuum, it is always 299,792 kilometers per second (186, 282 miles per second), a value denoted by the letter "c." According to Einstein's Theory of Relativity, nothing in the universe travels faster than this.

Refraction and Rainbows

Light travels slower in a medium than it does in a vacuum, and the speed is proportional to the density of the medium. This speed variation causes light to bend at the interface of two media -- a phenomenon called refraction. The angle at which it bends depends on the densities of the two media and the wavelength of the incident light. When light incident on a transparent medium is composed of wave fronts of different wavelengths, each wave front bends at a different angle, and the result is a rainbow.

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About the Author

Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. He began writing online in 2010, offering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts.

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25.3: The Law of Refraction

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Learning Objectives

By the end of this section, you will be able to:

  • Determine the index of refraction, given the speed of light in a medium.

It is easy to notice some odd things when looking into a fish tank. For example, you may see the same fish appearing to be in two different places (Figure \(\PageIndex{1}\)). This is because light coming from the fish to us changes direction when it leaves the tank, and in this case, it can travel two different paths to get to our eyes. The changing of a light ray’s direction (loosely called bending) when it passes through variations in matter is called refraction . Refraction is responsible for a tremendous range of optical phenomena, from the action of lenses to voice transmission through optical fibers.

Definition: REFRACTION

The changing of a light ray’s direction (loosely called bending) when it passes through variations in matter is called refraction.

SPEED OF LIGHT

The speed of light \(c\) not only affects refraction, it is one of the central concepts of Einstein’s theory of relativity. As the accuracy of the measurements of the speed of light were improved, \(c\) was found not to depend on the velocity of the source or the observer. However, the speed of light does vary in a precise manner with the material it traverses. These facts have far-reaching implications, as we will see in "Special Relativity." It makes connections between space and time and alters our expectations that all observers measure the same time for the same event, for example. The speed of light is so important that its value in a vacuum is one of the most fundamental constants in nature as well as being one of the four fundamental SI units.

A person looks at a fish tank and he sees the same fish in two different directions at the edge of the water tank facing him.

Why does light change direction when passing from one material (medium) to another? It is because light changes speed when going from one material to another. So before we study the law of refraction, it is useful to discuss the speed of light and how it varies in different media.

The Speed of Light

Early attempts to measure the speed of light, such as those made by Galileo, determined that light moved extremely fast, perhaps instantaneously. The first real evidence that light traveled at a finite speed came from the Danish astronomer Ole Roemer in the late 17th century. Roemer had noted that the average orbital period of one of Jupiter’s moons, as measured from Earth, varied depending on whether Earth was moving toward or away from Jupiter. He correctly concluded that the apparent change in period was due to the change in distance between Earth and Jupiter and the time it took light to travel this distance. From his 1676 data, a value of the speed of light was calculated to be \(2.26 \times 10^{8} m/s\) (only 25% different from today’s accepted value). In more recent times, physicists have measured the speed of light in numerous ways and with increasing accuracy. One particularly direct method, used in 1887 by the American physicist Albert Michelson (1852–1931), is illustrated in Figure \(\PageIndex{2}\). Light reflected from a rotating set of mirrors was reflected from a stationary mirror 35 km away and returned to the rotating mirrors. The time for the light to travel can be determined by how fast the mirrors must rotate for the light to be returned to the observer’s eye.

In stage one of the figure, the light falling from a source on an eight-sided mirror is viewed by an observer; in stage two, the mirror is made to rotate and the reflected light falling onto a stationary mirror kept at a certain distance of 35 kilometers is viewed by an observer. In stage three, the observer can see the reflected ray only when the mirror has rotated into the correct position just as the ray returns.

The speed of light is now known to great precision. In fact, the speed of light in a vacuum \(c\) is so important that it is accepted as one of the basic physical quantities and has the fixed value.

VALUE OF THE SPEED OF LIGHT

\[\begin{align} c &\equiv 2.99792458 \times 10^{8} \\[5pt] &\sim 3.00 \times 10^{8} m/s \end{align}\]

The approximate value of \(3.00 \times 10^{8} m/s\) is used whenever three-digit accuracy is sufficient. The speed of light through matter is less than it is in a vacuum, because light interacts with atoms in a material. The speed of light depends strongly on the type of material, since its interaction with different atoms, crystal lattices, and other substructures varies.

Definition: INDEX OF REFRACTION

We define the index of refraction \(n\) of a material to be

\[n = \frac{c}{v}, \label{index}\]

where \(v\) is the observed speed of light in the material. Since the speed of light is always less than \(c\) in matter and equals \(c\) only in a vacuum, the index of refraction is always greater than or equal to one. That is, \(n \gt 1\).

Table \(\PageIndex{1}\) gives the indices of refraction for some representative substances. The values are listed for a particular wavelength of light, because they vary slightly with wavelength. (This can have important effects, such as colors produced by a prism.) Note that for gases, \(n\) is close to 1.0. This seems reasonable, since atoms in gases are widely separated and light travels at \(c\) in the vacuum between atoms. It is common to take \(n = 1\) for gases unless great precision is needed. Although the speed of light \( v\) in a medium varies considerably from its value \( c\) in a vacuum, it is still a large speed.

Example \(\PageIndex{1}\): Speed of Light in Matter

Calculate the speed of light in zircon, a material used in jewelry to imitate diamond.

The speed of light in a material, \(v\), can be calculated from the index of refraction \(n\) of the material using the equation \(n = c/v\).

The equation for index of refraction (Equation \ref{index}) can be rearranged to determine \(v\)

\[v = \frac{c}{n}. \nonumber\]

The index of refraction for zircon is given as 1.923 in Table \(\PageIndex{1}\), and \(c\) is given in the equation for speed of light. Entering these values in the last expression gives

\[ \begin{align*} v &= \frac{3.00 \times 10^{8} m/s}{1.923} \\[5pt] &= 1.56 \times 10^{8} m/s. \end{align*}\]

Discussion:

This speed is slightly larger than half the speed of light in a vacuum and is still high compared with speeds we normally experience. The only substance listed in Table \(\PageIndex{1}\) that has a greater index of refraction than zircon is diamond. We shall see later that the large index of refraction for zircon makes it sparkle more than glass, but less than diamond.

Law of Refraction

Figure \(\PageIndex{3}\) shows how a ray of light changes direction when it passes from one medium to another. As before, the angles are measured relative to a perpendicular to the surface at the point where the light ray crosses it. (Some of the incident light will be reflected from the surface, but for now we will concentrate on the light that is transmitted.) The change in direction of the light ray depends on how the speed of light changes. The change in the speed of light is related to the indices of refraction of the media involved. In the situations shown in Figure \(\PageIndex{3}\), medium 2 has a greater index of refraction than medium 1. This means that the speed of light is less in medium 2 than in medium 1. Note that as shown in Figure \(\PageIndex{3a}\), the direction of the ray moves closer to the perpendicular when it slows down. Conversely, as shown in Figure \(\PageIndex{3b}\), the direction of the ray moves away from the perpendicular when it speeds up. The path is exactly reversible. In both cases, you can imagine what happens by thinking about pushing a lawn mower from a footpath onto grass, and vice versa. Going from the footpath to grass, the front wheels are slowed and pulled to the side as shown. This is the same change in direction as for light when it goes from a fast medium to a slow one. When going from the grass to the footpath, the front wheels can move faster and the mower changes direction as shown. This, too, is the same change in direction as for light going from slow to fast.

The figures compare the working of a lawn mower to that of the refraction phenomenon. In figure (a) the lawn mower goes from a sidewalk to grass, it slows down and bends towards a perpendicular drawn at the point of contact of the mower with the surface of separation. An imaginary line along the mower when it is on sidewalk is taken to be the incident ray and the angle which the mower makes with the perpendicular is taken to be theta one. As it goes into the grass, the mower turns and the imaginary line moves towards the perpendicular line drawn and makes an angle theta two with it. The imaginary line drawn along the mower when the mower is in the grass is taken to be the refracted ray. Sidewalk is taken to be a medium of refractive index n one and that of grass to be taken as n two. In figure (b), the situation is the reverse of what has happened in figure (a). The mower moves from grass to sidewalk and the ray of light moves away from the perpendicular when it speeds up.

The amount that a light ray changes its direction depends both on the incident angle and the amount that the speed changes. For a ray at a given incident angle, a large change in speed causes a large change in direction, and thus a large change in angle. The exact mathematical relationship is the law of refraction , or "Snell's Law," which is stated in equation form as

THE LAW OF REFRACTION (Snell's Law)

\[n_{1} \sin \theta_{1} = n_{2} \sin \theta_{2}.\label{25.4.2}\]

Here, \(n_{1}\) and \(n_{2}\) are the indices of refraction for medium 1 and 2, and \(\theta_{1}\) and \(\theta_{2}\) are the angles between the rays and the perpendicular in medium 1 and 2, as shown in Figure \(\PageIndex{3}\). The incoming ray is called the incident ray and the outgoing ray the refracted ray, and the associated angles the incident angle and the refracted angle. The law of refraction is also called Snell’s law after the Dutch mathematician Willebrord Snell (1591–1626), who discovered it in 1621. Snell’s experiments showed that the law of refraction was obeyed and that a characteristic index of refraction \(n\) could be assigned to a given medium. Snell was not aware that the speed of light varied in different media, but through experiments he was able to determine indices of refraction from the way light rays changed direction.

TAKE-HOME EXPERIMENT: A BROKEN PENCIL

A classic observation of refraction occurs when a pencil is placed in a glass half filled with water. Do this and observe the shape of the pencil when you look at the pencil sideways, that is, through air, glass, water. Explain your observations. Draw ray diagrams for the situation.

Example \(\PageIndex{2}\): Determine the Index of Refraction from Refraction Data

Find the index of refraction for medium 2 in Figure \(\PageIndex{3a}\), assuming medium 1 is air and given the incident angle is \(30.0^{\circ}\) and the angle of refraction is \(22.0^{\circ}\).

The index of refraction for air is taken to be 1 in most cases (and up to four significant figures, it is 1.000). Thus \(n_{1} = 1.00\) here. From the given information, \(\theta_{1} = 30.0^{\circ}\) and \(\theta_{2} = 22.0^{\circ}\) With this information, the only unknown in Snell’s law is \(n_{2}\), so that it can be used to find this unknown.

Snell's law (Equation \ref{25.4.2}) can be rearranged to isolate \(n_{2}\) gives

\[n_{2} = n_{1}\frac{\sin{\theta_{1}}}{\sin{\theta_{2}}}.\]

Entering known values,

\[ \begin{align*} n_{2} &= n_{1}\frac{\sin{30.0^{\circ}}}{\sin{22.0^{\circ}}} \\[5pt] &= \frac{0.500}{0.375} \\[5pt] &=1.33. \end{align*}\]

This is the index of refraction for water, and Snell could have determined it by measuring the angles and performing this calculation. He would then have found 1.33 to be the appropriate index of refraction for water in all other situations, such as when a ray passes from water to glass. Today we can verify that the index of refraction is related to the speed of light in a medium by measuring that speed directly.

Example \(\PageIndex{3}\): A Larger Change in Direction

Suppose that in a situation like that in the previous example, light goes from air to diamond and that the incident angle is \(30.0^{\circ}\). Calculate the angle of refraction \(\theta_{2}\) in the diamond.

Again the index of refraction for air is taken to be \(n_{1} = 1.00\), and we are given \(\theta_{1} = 30.0^{\circ}\). We can look up the index of refraction for diamond in Table \(\PageIndex{1}\), finding \(n_{2} = 2.419\). The only unknown in Snell’s law is \(\theta_{2}\), which we wish to determine.

Solving Snell’s law (Equation \ref{25.4.2}) for \(\sin{\theta_{2}}\) yields

\[ \sin{\theta_{2}} = \frac{n_{1}}{n_{2}}\sin{\theta_{1}}.\]

\[ \begin{align*} \sin{\theta_{2}} &= \frac{1.00}{2.419} \sin{30.0^{\circ}} \\[5pt] &= \left( 0.413 \right) \left( 0.500 \right) \\[5pt] &= 0.207. \end{align*}\]

The angle is thus

\[\theta_{2} = \sin{0.207}^{-1} = 11.9^{\circ}.\]

For the same \(30^{\circ}\) angle of incidence, the angle of refraction in diamond is significantly smaller than in water (\(11.9^{\circ}\) rather than \(22^{\circ}\) -- see the preceding example).

  • The changing of a light ray’s direction when it passes through variations in matter is called refraction.
  • The speed of light in vacuuum \(c = 2.99792458 \times 10^{8} \sim 3.00 \times 10^{8} m/s\)
  • Index of refraction \(n = \frac{c}{v}\), where \(v\) is the speed of light in the material, \(c\) is the speed of light in vacuum, and \(n\) is the index of refraction.
  • Snell’s law, the law of refraction, is stated in equation form as \(n_{1} \sin_{\theta_{1}} = n_{2} \sin_{\theta_{2}}\).

16.1 Reflection

Section learning objectives.

By the end of this section, you will be able to do the following:

  • Explain reflection from mirrors, describe image formation as a consequence of reflection from mirrors, apply ray diagrams to predict and interpret image and object locations, and describe applications of mirrors
  • Perform calculations based on the law of reflection and the equations for curved mirrors

Teacher Support

The learning objectives in this section help your students master the following standards:

  • (D) investigate behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler effect;
  • (E) describe and predict image formation as a consequence of reflection from a plane mirror and refraction through a thin convex lens; and
  • (F) describe the role of wave characteristics and behaviors in medical and industrial applications.

In addition, the High School Physics Laboratory Manual addresses content in this section in the lab titled: Mirrors and Lenses, as well as the following standards:

  • (D) investigate behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler effect.

Section Key Terms

Characteristics of mirrors.

[BL] Recall that, in geometry, angles are numbers that tell how far two straight lines are spread apart. The lines must be straight lines for the number to have meaning.

[OL] Geometry is the study of relationships involving points, lines, angles, and shapes. In this chapter, we are focused on the first three ideas.

[AL] In this chapter, we apply equations that use trigonometric functions that describe the properties of angles. Trigonometric functions are ratios of the lengths of two sides of a right triangle. There are six possible ratios; therefore, there are six such functions.

There are three ways, as shown in Figure 16.2 , in which light can travel from a source to another location. It can come directly from the source through empty space, such as from the Sun to Earth. Light can travel to an object through various media, such as air and glass. Light can also arrive at an object after being reflected, such as by a mirror. In all these cases, light is modeled as traveling in a straight line, called a ray . Light may change direction when it encounters the surface of a different material (such as a mirror) or when it passes from one material to another (such as when passing from air into glass). It then continues in a straight line—that is, as a ray. The word ray comes from mathematics. Here it means a straight line that originates from some point. It is acceptable to visualize light rays as laser rays (or even science fiction depictions of ray guns ).

Because light moves in straight lines, that is, as rays, and changes directions when it interacts with matter, it can be described through geometry and trigonometry. This part of optics, described by straight lines and angles, is therefore called geometric optics . There are two laws that govern how light changes direction when it interacts with matter: the law of reflection , for situations in which light bounces off matter; and the law of refraction , for situations in which light passes through matter. In this section, we consider the geometric optics of reflection .

[BL] Explain that light bounces is a simplification. The geometry of the path of a bouncing ball is similar to that of light, but what happens at the point of impact is different at the molecular level.

[OL] Indicate that the terms right angle , perpendicular , and normal line all mean the same thing: a vertical line at a 90° angle to a flat surface.

[AL] Recount and explain all the possible interactions of light with matter. Light can be absorbed at the surface of an opaque object. Some colors of light may be absorbed and others reflected. Light often is partially absorbed and partially reflected. It may also be transmitted through a transparent material, such as water or glass. Typically, if the surface of a transparent material is smooth, such as that of a window pane, light is transmitted partially and reflected partially.

Whenever we look into a mirror or squint at sunlight glinting from a lake, we are seeing a reflection. How does the reflected light travel from the object to your eyes? The law of reflection states: The angle of reflection , θ r θ r , equals the angle of incidence , θ i θ i . This law governs the behavior of all waves when they interact with a smooth surface, and therefore describe the behavior of light waves as well. The reflection of light is simplified when light is treated as a ray. This concept is illustrated in Figure 16.3 , which also shows how the angles are measured relative to the line perpendicular to the surface at the point where the light ray strikes it. This perpendicular line is also called the normal line , or just the normal . Light reflected in this way is referred to as specular (from the Latin word for mirror : speculum ).

We expect to see reflections from smooth surfaces, but Figure 16.4 , illustrates how a rough surface reflects light. Because the light is reflected from different parts of the surface at different angles, the rays go in many different directions, so the reflected light is diffused . Diffused light allows you to read a printed page from almost any angle because some of the rays go in different directions. Many objects, such as people, clothing, leaves, and walls, have rough surfaces and can be seen from many angles. A mirror, on the other hand, has a smooth surface and reflects light at specific angles.

When we see ourselves in a mirror, it appears that our image is actually behind the mirror. We see the light coming from a direction determined by the law of reflection. The angles are such that our image is exactly the same distance behind the mirror, d i , as the distance we stand away from the mirror, d o . Although these mirror images make objects appear to be where they cannot be (such as behind a solid wall), the images are not figments of our imagination. Mirror images can be photographed and videotaped by instruments and look just as they do to our eyes, which are themselves optical instruments. An image in a mirror is said to be a virtual image , as opposed to a real image . A virtual image is formed when light rays appear to diverge from a point without actually doing so.

Figure 16.5 helps illustrate how a flat mirror forms an image. Two rays are shown emerging from the same point, striking the mirror, and reflecting into the observer’s eye. The rays can diverge slightly, and both still enter the eye. If the rays are extrapolated backward, they seem to originate from a common point behind the mirror, allowing us to locate the image. The paths of the reflected rays into the eye are the same as if they had come directly from that point behind the mirror. Using the law of reflection—the angle of reflection equals the angle of incidence—we can see that the image and object are the same distance from the mirror. This is a virtual image, as defined earlier.

Fun In Physics

Mirror mazes.

Figure 16.6 is a chase scene from an old silent film called The Circus , starring Charlie Chaplin. The chase scene takes place in a mirror maze. You may have seen such a maze at an amusement park or carnival. Finding your way through the maze can be very difficult. Keep in mind that only one image in the picture is real—the others are virtual.

One of the earliest uses of mirrors for creating the illusion of space is seen in the Palace of Versailles, the former home of French royalty. Construction of the Hall of Mirrors ( Figure 16.7 ) began in 1678. It is still one of the most popular tourist attractions at Versailles.

Grasp Check

Only one Charlie in this image ( Figure 16.8 ) is real. The others are all virtual images of him. Can you tell which is real? Hint—His hat is tilted to one side.

  • The virtual images have their hats tilted to the right.
  • The virtual images have their hats tilted to the left.
  • The real images have their hats tilted to the right.
  • The real images have their hats tilted to the left.

Watch Physics

Virtual image.

This video explains the creation of virtual images in a mirror. It shows the location and orientation of the images using ray diagrams, and relates the perception to the human eye.

  • The distances of the image and the object from the mirror are the same.
  • The distances of the image and the object from the mirror are always different.
  • The image is formed at infinity if the object is placed near the mirror.
  • The image is formed near the mirror if the object is placed at infinity.

Have students construct a ray diagram for an object reflected in a plane mirror. Point out to them that all information can be represented in the diagram by using just paper, a pencil, a ruler, and a protractor. Students may use the preceding video and Figure 16.5 to help them to draw the necessary rays for the diagram. Have them compare the position and orientation of the virtual image with that of the object, paying particular attention to the identical distances that the object and image have with respect to the mirror surface.

Misconception Alert

[BL] Ask students to define virtual and dispel any misconceptions. Explain the term in relation to geometric optics.

[OL] Explain that a real focal point is a point at which there is a concentration of light energy that can be transformed into other useful forms. At a virtual focal point , on the other hand, light energy cannot be concentrated because no light actually goes to that point.

[AL] Explain the difference between a parabolic shape and a spherical shape. Use drawings of a cross-section of each. Point out that, for a short section of a curved mirror with very little curvature, a spherical mirror approximates a parabolic one.

Some mirrors are curved instead of flat. A mirror that curves inward is called a concave mirror , whereas one that curves outward is called a convex mirror . Pick up a well-polished metal spoon and you can see an example of each type of curvature. The side of the spoon that holds the food is a concave mirror; the back of the spoon is a convex mirror. Observe your image on both sides of the spoon.

Tips For Success

You can remember the difference between concave and convex by thinking, Concave means caved in .

Ray diagrams can be used to find the point where reflected rays converge or appear to converge, or the point from which rays appear to diverge. This is called the focal point , F. The distance from F to the mirror along the central axis (the line perpendicular to the center of the mirror’s surface) is called the focal length , f . Figure 16.9 shows the focal points of concave and convex mirrors.

Images formed by a concave mirror vary, depending on which side of the focal point the object is placed. For any object placed on the far side of the focal point with respect to the mirror, the rays converge in front of the mirror to form a real image, which can be projected onto a surface, such as a screen or sheet of paper However, for an object located inside the focal point with respect to the concave mirror, the image is virtual. For a convex mirror the image is always virtual—that is, it appears to be behind the mirror. The ray diagrams in Figure 16.10 show how to determine the nature of the image formed by concave and convex mirrors.

The information in Figure 16.10 is summarized in Table 16.1 .

Concave and Convex Mirrors

  • Silver spoon and silver polish, or a new spoon made of any shiny metal

Instructions

  • Choose any small object with a top and a bottom, such as a short nail or tack, or a coin, such as a quarter. Observe the object’s reflection on the back of the spoon.
  • Observe the reflection of the object on the front (bowl side) of the spoon when held away from the spoon at a distance of several inches.
  • Observe the image while slowly moving the small object toward the bowl of the spoon. Continue until the object is all the way inside the bowl of the spoon.
  • You should see one point where the object disappears and then reappears. This is the focal point.

Describe the differences in the image of the object on the two sides of the focal point. Explain the change. Identify which of the images you saw were real and which were virtual.

  • The height of the image became infinite.
  • The height of the object became zero.
  • The intensity of intersecting light rays became zero.
  • The intensity of intersecting light rays increased.

[BL] [OL] Ask students to identify as many examples as they can of curved mirrors that are used in everyday applications. Supply any they miss: security mirrors, mirrors for entering and exiting a driveway with poor visibility, rear-view mirrors, mirrors for application of cosmetics, and so on.

Parabolic Mirrors and Real Images

This video uses ray diagrams to show the special feature of parabolic mirrors that makes them ideal for either projecting light energy in parallel rays, with the source being at the focal point of the parabola, or for collecting at the focal point light energy from a distant source.

  • The rays do not polarize after reflection.
  • The rays are dispersed after reflection.
  • The rays are polarized after reflection.
  • The rays become parallel after reflection.

Teacher Demonstration

Have students use the demonstration in the video to construct a ray diagram that shows that rays from an object (upright arrow) placed at the focal point of a concave mirror emerge parallel to the central axis.

You should be able to notice everyday applications of curved mirrors. One common example is the use of security mirrors in stores, as shown in Figure 16.11 .

Some telescopes also use curved mirrors and no lenses (except in the eyepieces) both to magnify images and to change the path of light. Figure 16.12 shows a Schmidt-Cassegrain telescope. This design uses a spherical primary concave mirror and a convex secondary mirror. The image is projected onto the focal plane by light passing through the perforated primary mirror. The effective focal length of such a telescope is the focal length of the primary mirror multiplied by the magnification of the secondary mirror. The result is a telescope with a focal length much greater than the length of the telescope itself.

A parabolic concave mirror has the very useful property that all light from a distant source, on reflection by the mirror surface, is directed to the focal point. Likewise, a light source placed at the focal point directs all the light it emits in parallel lines away from the mirror. This case is illustrated by the ray diagram in Figure 16.13 . The light source in a car headlight, for example, is located at the focal point of a parabolic mirror.

Parabolic mirrors are also used to collect sunlight and direct it to a focal point, where it is transformed into heat, which in turn can be used to generate electricity. This application is shown in Figure 16.14 .

The Application of the Curved Mirror Equations

[BL] [OL] Review operations for manipulating fractions and for rearranging equations involving fractional values of variables.

[AL] Demonstrate how to solve equations of the type 1 a = 1 b + 1 c 1 a = 1 b + 1 c for any of the variables in terms of the other two. Rearrange so that the variable solved for is not a reciprocal.

Curved mirrors and the images they create involve a fairly small number of variables: the mirror’s radius of curvature, R ; the focal length, f ; the distances of the object and image from the mirror, d o and d i , respectively; and the heights of the object and image, h o and h i , respectively. The signs of these values indicate whether the image is inverted, erect (upright), real, or virtual. We now look at the equations that relate these variables and apply them to everyday problems.

Figure 16.15 shows the meanings of most of the variables we will use for calculations involving curved mirrors.

The basic equation that describes both lenses and mirrors is the lens/mirror equation

This equation can be rearranged several ways. For example, it may be written to solve for focal length.

Magnification, m , is the ratio of the size of the image, h i , to the size of the object, h o . The value of m can be calculated in two ways.

This relationship can be written to solve for any of the variables involved. For example, the height of the image is given by

We saved the simplest equation for last. The radius of curvature of a curved mirror, R , is simply twice the focal length.

We can learn important information from the algebraic sign of the result of a calculation using the previous equations:

  • A negative d i indicates a virtual image; a positive value indicates a real image
  • A negative h i indicates an inverted image; a positive value indicates an erect image
  • For concave mirrors, f is positive; for convex mirrors, f is negative

Now let’s apply these equations to solve some problems.

Worked Example

Calculating focal length.

A person standing 6.0 m from a convex security mirror forms a virtual image that appears to be 1.0 m behind the mirror. What is the focal length of the mirror?

The person is the object, so d o = 6.0 m. We know that, for this situation, d o is positive. The image is virtual, so the value for the image distance is negative, so d i = –1.0 m.

Now, use the appropriate version of the lens/mirror equation to solve for focal length by substituting the known values.

f = d i d o d o + d i = ( − 1.0 ) ( 6.0 ) 6.0 + ( − 1.0 ) = − 6.0 5.0 = − 1.2  m f = d i d o d o + d i = ( − 1.0 ) ( 6.0 ) 6.0 + ( − 1.0 ) = − 6.0 5.0 = − 1.2  m

The negative result is expected for a convex mirror. This indicates the focal point is behind the mirror.

Calculating Object Distance

Electric room heaters use a concave mirror to reflect infrared (IR) radiation from hot coils. Note that IR radiation follows the same law of reflection as visible light. Given that the mirror has a radius of curvature of 50.0 cm and produces an image of the coils 3.00 m in front of the mirror, where are the coils with respect to the mirror?

We are told that the concave mirror projects a real image of the coils at an image distance d i = 3.00 m. The coils are the object, and we are asked to find their location—that is, to find the object distance d o . We are also given the radius of curvature of the mirror, so that its focal length is f = R /2 = 25.0 cm (a positive value, because the mirror is concave, or converging). We can use the lens/mirror equation to solve this problem.

Because d i and f are known, the lens/mirror equation can be used to find d o .

Rearranging to solve for d o , we have

Entering the known quantities gives us

Note that the object (the coil filament) is farther from the mirror than the mirror’s focal length. This is a case 1 image ( d o > f and f positive), consistent with the fact that a real image is formed. You get the most concentrated thermal energy directly in front of the mirror and 3.00 m away from it. In general, this is not desirable because it could cause burns. Usually, you want the rays to emerge parallel, and this is accomplished by having the filament at the focal point of the mirror.

Note that the filament here is not much farther from the mirror than the focal length, and that the image produced is considerably farther away.

Practice Problems

What is the focal length of a makeup mirror that produces a magnification of 1.50 when a person’s face is 12.0 cm away? Construct a ray diagram using paper, a pencil and a ruler to confirm your calculation.

Check Your Understanding

Use these questions to assess student achievement of the section’s learning objectives. If students are struggling with a specific objective, these questions will help identify which one, and then you can direct students to the relevant content.

How does the object distance, d o , compare with the focal length, f, for a concave mirror that produces an image that is real and inverted?

  • d o > f, where d o and f are object distance and focal length, respectively.
  • d o < f, where d o and f are object distance and focal length, respectively.
  • d o = f, where do and f are object distance and focal length, respectively.
  • d o = 0, where do is the object distance.

Use the law of reflection to explain why it is not a good idea to polish a mirror with coarse sandpaper.

  • The surface becomes smooth. A smooth surface produces a sharp image.
  • The surface becomes irregular. An irregular surface produces a sharp image.
  • The surface becomes smooth. A smooth surface transmits but does not reflect light.
  • The surface becomes irregular. An irregular surface produces a blurred image.
  • It is real and erect.
  • It is real and inverted.
  • It is virtual and inverted.
  • It is virtual and erect.

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May 20, 2016

How does light travel?

by Matt Williams, Universe Today

How does light travel?

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century's BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern, the 20th century led to breakthroughs that showed that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein's Theory of Relativity. However, there remains many fascinating and unanswered questions when it comes to light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light in the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle's theory of light, which viewed it as being a disturbance in the air (one of his four "elements" that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or "corpuscles"). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton.

Newton's corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise "Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light". According to Newton, the principles of light could be summed as follows:

  • Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
  • These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to "wave theory", which had been advocated by 17th century Dutch astronomer Christiaan Huygens. . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his "Traité de la lumière" ("Treatise on Light"). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment, which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young's version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation , would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter's moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century , James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell's equations) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c).

In 1905, Albert Einstein published "On the Electrodynamics of Moving Bodies", in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity, which reconciled Maxwell's equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

How does light travel?

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect, Einstein based his idea on Planck's earlier work with "black bodies" – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein's photoelectric effect was attempt to explain the "black body problem", in which a black body emits electromagnetic radiation due to the object's heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein's explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named "photons". For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg's "uncertainty principle" (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger's paradox that claimed that all particles have a " wave function ".

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly "collapse", or rather "decohere", to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the "Schrödinger's Cat" paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly slow down or arrest the speed of light is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

Source: Universe Today

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NOTIFICATIONS

Light travels in straight lines outwards from its source.

  • + Create new collection

Rays of golden sunlight through clouds

Light is produced by a source and travels outwards from the source in straight lines and all directions at 300,000 km/s.

Because we can’t actually see light travelling, we depict this property in diagrams that show light travelling in straight lines called rays. These rays travel out from the source until they hit something. Depending on the properties of whatever they hit, all or some of the rays will pass through it, bounce off it or be absorbed by it.

Only a vacuum allows the completely free passage of light. Some light energy is always absorbed by any material through which light passes. Thicker samples of the same materials absorb more energy, for example, objects are seen more clearly through a thin layer of glass than through a solid glass block.

Related articles

  • Light and shadows
  • Light basics
  • Alternative conceptions about light
  • Reflection of light
  • Refraction of light

Related images

  • The Sun’s rays
  • The Sun appearing in the east

Related activity

  • Investigating shadows

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light travel diagram

Shadows: Effects of the absence of light

This interactive explores the sequential and interlinking science concepts that underpin knowledge and understanding about light and shadows.

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How does light travel?

Gretchen Walker, Patricia McGlashan, Laura Danly, Eric Hamilton, Stephanie Fotiadi, The Education Department at the American Museum of Natural History

light travel diagram

This activity includes two experiments that explore shadows and light and how mirrors can demonstrate how light travels.

Notes from our reviewers

The CLEAN collection is hand-picked and rigorously reviewed for scientific accuracy and classroom effectiveness. Read what our review team had to say about this resource below or learn more about how CLEAN reviews teaching materials .

  • Teaching Tips The document is targeted to after school groups that are generally multi-aged. There may be additional preparations for materials that offer extensions for the older groups. This is one activity in a large educator guide about the sun and light. This activity can be related back to the climate and earth's processes, while the other activities are more solar system/universe based. This activity is pages 15-17 and no other reference is neccessary.
  • About the Science Using flashlights and mirrors, this experiment demonstrates the basics of the physics of light. It establishes the concepts of how light travels and how it interacts with objects, for example by casting shadows or being reflected. This activity can be extended to relate to solar energy. This is very hands-on. You can propose a hypothesis and test it. Passed initial science review - expert science review pending.
  • About the Pedagogy This experiment has a hands-on activity for the students to learn about light. The activities and outlines are rendered in an organized and consistent manner both structurally and graphically. There are also several discussion questions provided and the students are asked to record their observations in a science journal.
  • Technical Details/Ease of Use Overview, connections, and activity prep and process is well organized for all activities. Most of the included links are broken. However the content can be found easily with a quick online search.
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light travel diagram

by Chris Woodford . Last updated: December 15, 2021.

W ere you ever scared of the dark? It's not surprising if you were, or if you still are today, because humans are creatures of the light , deeply programmed through millions of years of history to avoid the dark dangers of the night. Light is vitally important to us, but we don't always take the trouble to understand it. Why does it make some things appear to be different colors from others? Does it travel as particles or as waves? Why does it move so quickly? Let's take a closer look at some of these questions—let's shed some light on light!

Photo: Ordinary light looks white, but if you shine it through a prism (wedge) of glass, you can see that it's really made from a whole spectrum of colors.

What is light?

When we're very young, we have a very simple idea about light: the world is either light or dark and we can change from one to the other just by flicking a switch on the wall. But we soon learn that light is more complex than this.

Light arrives on our planet after a speedy trip from the Sun, 149 million km (93 million miles away). Light travels at 186,000 miles (300,000 km) per second, so the light you're seeing now was still tucked away in the Sun about eight minutes ago. Put it another way, light takes roughly twice as long to get from the Sun to Earth as it does to make a cup of coffee!

Light is a kind of energy

But why does light make this journey at all? As you probably know, the Sun is a nuclear fireball spewing energy in all directions. The light that we see it simply the one part of the energy that the Sun makes that our eyes can detect. When light travels between two places (from the Sun to the Earth or from a flashlight to the sidewalk in front of you on a dark night), energy makes a journey between those two points. The energy travels in the form of waves (similar to the waves on the sea but about 100 million times smaller)—a vibrating pattern of electricity and magnetism that we call electromagnetic energy . If our eyes could see electricity and magnetism, we might see each ray of light as a wave of electricity vibrating in one direction and a wave of magnetism vibrating at right angles to it. These two waves would travel in step and at the speed of light.

Picture: Light energy likes to travel outwards! Most natural light floods into our world from the Sun, shown here in a dramatic closeup, emitting a blast of radiation called a solar flare. Photo courtesy of NASA Solar Dynamics Observatory (SDO) .

Is light a particle or a wave?

For hundreds of years, scientists have argued over whether light is really a wave at all. Back in the 17th century, the brilliant English scientist Sir Isaac Newton (1642–1727)—one of the first people to study the matter in detail—thought light was a stream of "corpuscles" or particles . But his great rival, a no-less-brilliant Dutchman named Christiaan Huygens (1629–1695), was quite adamant that light was made up of waves .

Photo: Isaac Newton argued that light was a stream of particles. Picture by William Thomas Fry courtesy of US Library of Congress .

Thus began a controversy that still rumbles on today—and it's easy to see why. In some ways, light behaves just like a wave: light reflects off a mirror , for example, in exactly the same way that waves crashing in from the sea "reflect" off sea walls and go back out again. In other ways, light behaves much more like a stream of particles—like bullets firing in rapid succession from a gun. During the 20th century, physicists came to believe that light could be both a particle and a wave at the same time. (This idea sounds quite simple, but goes by the rather complex name of wave-particle duality .)

The real answer to this problem is more a matter of philosophy and psychology than physics. Our understanding of the world is based on the way our eyes and brains interpret it. Sometimes it seems to us that light is behaving like a wave; sometimes it seems like light is a stream of particles. We have two mental pigeonholes and light doesn't quite fit into either of them. It's like the glass slipper that doesn't fit either of the ugly sisters (particle or wave). We can pretend it nearly fits both of them, some of the time. But in truth, light is simply what it is—a form of energy that doesn't neatly match our mental scheme of how things should be. One day, someone will come up with a better way of describing and explaining it that makes perfect sense in all situations.

How light behaves

Light waves (let's assume they are indeed waves for now) behave in four particularly interesting and useful ways that we describe as reflection, refraction, diffraction, and interference.

The most obvious thing about light is that it will reflect off things. The only reason we can see the things around us is that light, either from the Sun or from something like an electric lamp here on Earth, reflects off them into our eyes. Cut off the source of the light or stop it from reaching your eyes and those objects disappear. They don't cease to exist, but you can no longer see them.

Photo: Now that's what I call a mirror! In fact, it's six segments of the huge mirror from the James Webb Space Telescope. Picture by courtesy of NASA .

Reflection can happen in two quite different ways. If you have a smooth, highly polished surface and you shine a narrow beam of light at it, you get a narrow beam of light reflected back off it. This is called specular reflection and it's what happens if you shine a flashlight or laser into a mirror: you get a well-defined beam of light bouncing back towards you. Most objects aren't smooth and highly polished: they're quite rough. So, when you shine light onto them, it's scattered all over the place. This is called diffuse reflection and it's how we see most objects around us as they scatter the light falling on them.

If you can see your face in something, it's specular reflection; if you can't see your face, it's diffuse reflection. Polish up a teaspoon and you can see your face quite clearly. But if the spoon is dirty, all the bits of dirt and dust are scattering light in all directions and your face disappears.

Photo: Laser beams bending (refracting) through a crystal. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL) .

Light waves travel in straight lines through empty space (a vacuum), but more interesting things happen to them when they travel through other materials—especially when they move from one material to another. That's not unusual: we do the same thing ourselves.

Have you noticed how your body slows down when you try to walk through water? You go racing down the beach at top speed but, as soon as you hit the sea, you slow right down. No matter how hard you try, you cannot run as quickly through water as through air. The dense liquid is harder to push out of the way, so it slows you down. Exactly the same thing happens to light if you shine it into water , glass , plastic or another more dense material: it slows down quite dramatically. This tends to make light waves bend—something we usually call refraction .

How refraction works

Photo: Refraction makes a drinking straw look bent (the top and bottom appear not to be joined up) when it's standing in a jug of water. .

You've probably noticed that water can bend light. You can see this for yourself by putting a straw in a glass of water. Notice how the straw appears to kink at the point where the water meets the air above it. The bending happens not in the water itself but at the junction of the air and the water. You can see the same thing happening in this photo of laser light beams shining between two crystals up above. As the beams cross the junction, they bend quite noticeably.

Why does this happen? You may have learned that the speed of light is always the same, but that's only true when light travels in a vacuum. In fact, light travels more slowly in some materials than others. It goes more slowly in water than in air. Or, to put it another way, light slows down when it moves from air to water and it speeds up when it moves from water to air. This is what causes the straw to look bent. Let's look into this a bit more closely.

Imagine a light ray zooming along through the air at an angle to some water. Now imagine that the light ray is actually a line of people swimming along in formation, side-by-side, through the air. The swimmers on one side are going to enter the water more quickly than the swimmers on the other side and, as they do so, they are going to slow down—because people move more slowly in water than in air. That means the whole line is going to start slowing down, beginning with the swimmers at one side and ending with the swimmers on the other side some time later. That's going to cause the entire line to bend at an angle. This is exactly how light behaves when it enters water—and why water makes a straw look bent.

Refraction is amazingly useful. If you wear eyeglasses, you probably know that the lenses they contain are curved-shape pieces of glass or plastic that bend (refract) the light from the things you're looking at. Bending the light makes it seem to come from nearer or further away (depending on the type of lenses you have), which corrects the problem with your sight. To put it another way, your eyeglasses fix your vision by slowing down incoming light so it shifts direction slightly. Binoculars , telescopes , cameras , night vision goggles , and many other things with lenses work in exactly the same way (collectively we call these things optical equipment ).

Although light normally travels in straight lines, you can make it bend round corners by shooting it down thin glass or plastic pipes called fiber-optic cables. Reflection and refraction are at work inside these "light pipes" to make rays of light follow an unusual path they wouldn't normally take.

Diffraction

We can hear sounds bending round doorways, but we can't see round corners—why is that? Like light, sound travels in the form of waves (they're very different kinds of waves, but the idea of energy traveling in a wave pattern is broadly the same). Sound waves tend to range in size from a few centimeters to a few meters, and they will spread out when they come to an opening that is roughly the same size as they are—something like a doorway, for example. If sound is rushing down a corridor in your general direction and there's a doorway opening onto the room where you're sitting, the sound waves will spread in through the doorway and travel to your ears. The same thing does not happen with light. But light will spread out in an identical way if you shine it on a tiny opening that's of roughly similar size to its wavelength. You may have noticed this effect, which is called diffraction , if you screw your eyes up and look at a streetlight in the dark. As your eyes close, the light seems to spread out in strange stripes as it squeezes through the narrow gaps between your eyelids and eyelashes. The tighter you close your eyes, the more the light spreads (until it disappears when you close your eyes completely).

Artwork: When light from a laser (1) passes through a narrow slit (2), the waves spread out (3) and form a diffraction pattern of light and dark bands (4). Different numbers, shapes, and sizes of slits produce more complex diffraction patterns.

Interference

If you stand above a calm pond (or a bath full of water) and dip your finger in (or allow a single drop to drip down to the water surface from a height), you'll see ripples of energy spreading outwards from the point of the impact. If you do this in two different places, the two sets of ripples will move toward one another, crash together, and form a new pattern of ripples called an interference pattern. Light behaves in exactly the same way. If two light sources produce waves of light that travel together and meet up, the waves will interfere with one another where they cross. In some places the crests of waves will reinforce and get bigger, but in other places the crest of one wave will meet the trough of another wave and the two will cancel out.

Photo: Thin-film interference makes the colors you see swirling around on the surface of soap bubbles.

Interference causes effects like the swirling, colored spectrum patterns on the surface of soap bubbles and the similar rainbow effect you can see if you hold a compact disc up to the light. What happens is that two reflected light waves interfere. One light wave reflects from the outer layer of the soap film that wraps around the air bubble, while a second light wave carries on through the soap, only to reflect off its inner layer. The two light waves travel slightly different distances so they get out of step. When they meet up again on the way back out of the bubble, they interfere. This makes the color of the light change in a way that depends on the thickness of the soap bubble. As the soap gradually thins out, the amount of interference changes and the color of the reflected light changes too. Read more about this in our article on thin-film interference . Interference is very colorful, but it has practical uses too. A technique called interferometry can use interfering laser beams to measure incredibly small distances.

Where does light come from?

Photo: Arc welding gives off light when metals are melted by an electric current. The atoms are getting quite excited here! Picture by Martin Wright, courtesy of US Navy .

If you've read our article on energy , you'll know that energy is something that doesn't just turn up out of the blue: it has to come from somewhere. There is a fixed amount of energy in the Universe and no process ever creates or destroys energy—it simply turns some of the existing energy into one or more other forms. This idea is a basic law of physics called the conservation of energy and it applies to light as much as anything else. So where then does light comes from? How exactly do you "make" light?

It turns out that light is made inside atoms when they get "excited". That's not excited in the silly, giggling sense of the word, but in a more specialized scientific sense. Think of the electrons inside atoms as a bit like fireflies sitting on a ladder. When an atom absorbs energy, for one reason or another, the electrons get promoted to higher energy levels. Visualize one of the fireflies moving up to a higher rung on the ladder. Unfortunately, the ladder isn't quite so stable with the firefly wobbling about up there, so the fly takes very little persuading to leap back down to where it was before. In so doing, it has to give back the energy it absorbed—and it does that by flashing its tail.

That's pretty much what happens when an atom absorbs energy. An electron inside it jumps to a higher energy level, but makes the atom unstable. As the electron returns to its original level, it gives back the energy as a flash of light called a photon .

How atoms make light

Atoms are the tiny particles from which all things are made. Simplified greatly, an atom looks a bit like our solar system, which has the Sun at its center and planets orbiting around it. Most of the atom's mass is concentrated in the nucleus at the center (red), made from protons and neutrons packed together. Electrons (blue) are arranged around the nucleus in shells (sometimes called orbitals, or energy levels). The more energy an electron has, the farther it is from the nucleus.

Atoms make light in a three-step process:

  • They start off in their stable "ground state" with electrons in their normal places.
  • When they absorb energy, one or more electrons are kicked out farther from the nucleus into higher energy levels. We say the atom is now "excited."
  • However, an excited atom is unstable and quickly tries to get back to its stable, ground state. So it gives off the excess energy it originally gained as a photon of energy (wiggly line): a packet of light.

How light really works

Once you understand how atoms take in and give out energy, the science of light makes sense in a very interesting new way. Think about mirrors , for example. When you look at a mirror and see your face reflected, what's actually going on? Light (maybe from a window) is hitting your face and bouncing into the mirror. Inside the mirror, atoms of silver (or another very reflective metal) are catching the incoming light energy and becoming excited. That makes them unstable, so they throw out new photons of light that travel back out of the mirror towards you. In effect, the mirror is playing throw and catch with you using photons of light as the balls!

The same idea can help us explain things like photocopiers and solar panels (flat sheets of the chemical element silicon that turn sunlight into electricity). Have you ever wondered why solar panels look black even when they're in full sunlight? That's because they're reflecting back little or none of the light that falls on them and absorbing all the energy instead. (Things that are black absorb light, and reflect little or none, while things that are white reflect virtually all the light that falls on them, and absorb little or none. That's why it's best to wear white clothes on a scorching hot day.) Where does the energy go in a solar panel if it's not reflected? If you shine sunlight onto the solar cells in a solar panel, the atoms of silicon in the cells catch the energy from the sunlight. Then, instead of producing new photons, they produce a flow of electricity instead through what's known as the photoelectric (or photovoltaic) effect. In other words, the incoming solar energy (from the Sun) is converted to outgoing electricity.

Hot light and cold light

What would make an atom absorb energy in the first place? You might give it some energy by heating it up. If you put an iron bar in a blazing fire, the bar would eventually heat up so much that it glowed red hot. What's happening is that you're supplying energy to the iron atoms inside the bar and getting them excited. Their electrons are being promoted to higher energy levels and making the atoms unstable. As the electrons return to lower levels, they're giving off their energy as photons of red light—and that's why the bar seems to glow red. The fire gives off light for exactly the same reason.

Old-style electric lamps work this way too. They make light by passing electricity through a very thin wire filament so it gets incredibly hot. Excited atoms inside the hot filament turn the electrical energy passing through them into light you can see by constantly giving off photons. When we make light by heating things, that's called incandescence . So old-style lamps are sometimes called incandescent lamps .

You can also get atoms excited in other ways. Energy-saving light bulbs that use fluorescence are more energy efficient because they make atoms crash about and collide, making lots of light without making heat. In effect, they make cold light rather than the hot light produced by older-style, energy-wasting bulbs. Creatures like fireflies make their light through a chemical process using a substance called luciferin. The broad name for the various different ways of making light by exciting the atoms inside things is luminescence .

(Let's note in passing that light has some other interesting effects when it gets involved in chemistry. That's how photochromic sunglass lenses work.)

Light of many colors

Photo: A rainbow splits sunlight ("white" light) into its component colors because it bends different colors ( wavelengths of light ) by different amounts. Shorter wavelengths are bent more than longer wavelengths, so blue light is bent more than red. That's why blue is always on the inside of a rainbow and red is on the outside.

Color (spelled "colour" in the UK) is one of the strangest things about light. Here's one obvious riddle: if we see things because sunlight is reflected off them, how come everything isn't the same color? Why isn't everything the color of sunlight? You probably know the answer to this already. Sunlight isn't light of just one color—it's what we call white light, made up of all the different colors mixed together. We know this because we can see rainbows , those colorful curves that appear in the sky when droplets of water split sunlight into its component colors by refracting (bending) different colors of light by different amounts.

Why does a tomato look red? When sunlight shines on a tomato, the red part of the sunlight is reflected back again off the tomato's skin, while all the other colors of lights are absorbed (soaked into) the tomato, so you don't see them. That's just as true of a blue book, which reflects only the blue part of sunlight but absorbs light of other colors.

Why does a tomato appear red and not blue or green? Think back to how atoms make light. When sunlight falls on a tomato, the incoming light energy excites atoms in the tomato's skin. Electrons are promoted to higher energy levels to capture the energy, but soon fall back down again. As they do so, they give off photons of new light—and that just happens to correspond to the kind of light that our eyes see as red. Tomatoes, in other words, are like precise optical machines programmed to produce photons of red light when sunlight falls on them.

If you shone light of other colors on tomatoes, what would happen? Let's suppose you made some green light by passing sunlight through a piece of green plastic (something we call a filter ). If you shone this on a red tomato, the tomato would appear black. That's because tomatoes absorb green light. There is simply no red light for them to reflect.

Photo: A tomato reflects the red part of sunlight and absorbs all the other colors.

It's not how it is—it's how you see it

Many of the things we think are true of the world turn out to be true only of ourselves. We think tomatoes are red, but in fact we only see them that way. If our eyes were built differently, we might see the light photons that tomatoes produce as light of a totally different color. And there's no real way any of us can be sure that what we see as "red" is the same as what anyone else sees as red: there's no way to prove that my red is the same as yours. Some of the most interesting aspects of the things we see come down to the psychology of perception (how our eyes see the world and how our brains make sense of that), not the physics of light. Color blindness and optical illusions are two examples of this.

Understanding light is a brilliant example of what being a scientist is all about. Science isn't like other subjects. It's not like history (a collection of facts about past events) or law (the rights and wrongs of how people behave). It's an entirely different way of thinking about the world and making sense of it. When you understand the science of light, you feel you've turned part of the world inside out—you're looking from the inside, seeing everything in a totally new way, and understanding for the first time why it all makes sense. Science can throw a completely different light on the world—it can even throw light on light itself!

If you liked this article...

Find out more, on this website.

  • Fiber optics
  • Microscopes
  • Thermochromic materials
  • Thin-film interference

On other websites

  • Optics for kids : Simple and fun introductory site from the Optical Society of America.

For younger readers

  • The Illuminating World of Light with Max Axiom, Super Scientist by Emily Sohn and Nick Derington. Capstone, 2019. A 32-page, app-linked graphic novel (comic-style) introduction for ages 8–14, aimed at engaging reluctant readers who might not pick up an ordinary school science book.
  • Light in a Flash by Georgia Amson-Bradshaw. Rosen, 2019/Franklin Watts, 2017. Facts, quizzes, and experiments lighten this 32-page introduction for ages 7–9.
  • A Project Guide to Light and Optics by Colleen Kessler. Mitchell Lane, 2012. A hands-on, activity-led guide to light for ages 9–12.
  • Scientific Pathways: Light by Chris Woodford. Rosen, 2013. This is one of my own books, also for ages 9–12, and it briefly charts the history of our efforts to understand light (Previously published as Routes of Science: Light , Blackbirch, 2004.)
  • Horrible Science: Frightening Light by Nick Arnold. Scholastic, 1999. A 160-page, text-led read for ages 8–12.
  • Light by David Burnie. DK, 1998. One of the well-known DK Eyewitness books combining science, technology, and history in an easily digestible volume. Best for ages 9–12 (though interesting for older people too).

For older readers

General books.

  • Light Years: The Extraordinary Story of Mankind's Fascination with Light by Brian Clegg. Icon, 2015. A whistle-stop tour through the scientific history of light.
  • Light: A Very Short Introduction by Ian Walmsley. Oxford, 2015. A solid introduction that takes us (in order) through light rays, waves, duality, relativity, and quantum theory. Quite a lot is compressed into just over 100 pages so the going (for beginners) isn't always easy.
  • QED: The Strange Theory of Light and Matter by Richard P. Feynman. Penguin, 2007 (reprinted in numerous editions). One of the 20th-century's greatest physicists explains the interactions between light and electrons.
  • Optics by Eugene Hecht. Addison-Wesley, 2016. The classic undergraduate textbook on light and optics—and the one I used myself some years ago.
  • Optics by K.K.Sharma. Academic Press, 2006. An alternative textbook for students, but with more about optical applications.

Text copyright © Chris Woodford 2008, 2018. All rights reserved. Full copyright notice and terms of use .

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Optics (Essentials) - Class 12th

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How to Prove That Light Travels in a Straight Path

Last Updated: November 29, 2022 Fact Checked

This article was co-authored by Chris Hasegawa, PhD . Dr. Chris Hasegawa was a Science Professor and the Dean at California State University Monterey Bay. Dr. Hasegawa specializes in teaching complex scientific concepts to students. He holds a BS in Biochemistry, a Master’s in Education, and his teaching credential from The University of California, Davis. He earned his PhD in Curriculum and Instruction from The University of Oregon. Before becoming a professor, Dr. Hasegawa conducted biochemical research in Neuropharmacology at the National Institute of Health. He also taught physical and life sciences and served as a teacher and administrator at public schools in California, Oregon, and Arizona. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 209,697 times.

Light is an essential part of your day. It allows you to see objects, shapes, and colors. In fact, the pupils in your eyes filter in light to help you see everything around you. As part of a school assignment, you may be asked to prove that light travels in a straight line. You can do this using basic household items in three easy experiments.

Making a Light Pinhole

Step 1 Gather your materials.

  • Three index cards.
  • A piece of modeling clay or sticky tack. You can also use double sided tape.

Chris Hasegawa, PhD

  • A hole puncher.

Step 2 Punch a hole in the center of the index cards.

  • Take the hole puncher and punch a hole at the center of the card where the two lines intersect. Do this for the other two cards.

Step 3 Use the modeling clay to stand up the cards.

  • Form a stand for the cards using the clay so the cards are straight and upright. Use the ruler to ensure the cards are two to five inches from each other.
  • You can also use double sided tape to attach the cards to a surface in a vertical position. Do not cover or obstruct the hole in the center of the cards with modeling clay or tape.

Step 4 Position the flashlight or the laser pointer at one end of the row of cards.

  • Note that the light can be seen through all the holes. You should be able to see the light go through all the holes and land on a wall or surface beyond the last index card.

Step 5 Move the flashlight or laser pointer so it does not hit the center of the first card.

Using a Mirror and a Flashlight

Step 1 Gather your materials.

  • Two to three sheets of black paper.
  • Small objects like buttons, bottle caps, or dimes.

Step 2 Place the objects on the black paper.

  • The other person will use the small mirror to reflect the flashlight so it hits the objects. Move close to the light, at an angle, to catch the light so it hits the objects.
  • You may need to position more than one mirror to create a light path that shines on the objects. Play around with reflecting the light on the mirrors until the light hits the objects. You can also move the objects around the room to create a more complicated light path, using the flashlight as the light source.
  • This experiment shows that light travels in a straight line in the air. But it also bounces off of a reflective surface, like a mirror. The angle of the light as it bounces off the mirror will be the same as the angle of the light as it hits the mirror. The mirror reflects the light and changes its path from a straight line to an angled straight line.

Using Water and Oil

Step 1 Gather your materials.

  • A large glass jar.
  • Access to water.
  • One cup of oil.

Step 2 Pour water into the jar.

  • Make sure the jar is large enough to fit the ruler.

Step 3 Use a spoon to run the oil over the surface of the water.

  • Note that the numbers appear stretched or magnified as the light rays bend in the oil and the water. Move the ruler from side to side to note the different appearances of the ruler numbers in the oil and in the water.
  • This will show that light travels at different speeds in different mediums, such as air, oil, and water. It will travel in a straight line in the air, but it will bend when it changes speed due to contact with a certain medium, like oil or water.

Expert Q&A

Chris Hasegawa, PhD

Things You'll Need

  • A piece of modeling clay or sticky tack. You can also use tape.
  • A flashlight or a laser pointer.
  • A flashlight.
  • A small mirror.

You Might Also Like

Make a Faraday Cage

  • ↑ http://www.ducksters.com/science/experiment_light_travel.php
  • ↑ Chris Hasegawa, PhD. Retired Science Professor & Dean. Expert Interview. 29 July 2021.
  • ↑ https://www.science-sparks.com/science-fair-projects-light-maze/
  • ↑ https://www.scientificamerican.com/article/now-you-see-it-testing-out-light-refraction/

About This Article

Chris Hasegawa, PhD

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light travel diagram

How a Lens Refracts Light

First lets consider a double convex lens . Suppose that several rays of light approach the lens; and suppose that these rays of light are traveling parallel to the principal axis . Upon reaching the front face of the lens, each ray of light will refract towards the normal to the surface. At this boundary, the light ray is passing from air into a more dense medium (usually plastic or glass). Since the light ray is passing from a medium in which it travels fast (less optically dense ) into a medium in which it travels relatively slow (more optically dense ), it will bend towards the normal line. This is the FST principle of refraction . This is shown for two incident rays on the diagram below. Once the light ray refracts across the boundary and enters the lens, it travels in a straight line until it reaches the back face of the lens. At this boundary, each ray of light will refract away from the normal to the surface. Since the light ray is passing from a medium in which it travels slow (more optically dense ) to a medium in which it travels fast (less optically dense ), it will bend away from the normal line; this is the SFA principle of refraction .

The above diagram shows the behavior of two incident rays approaching parallel to the principal axis. Note that the two rays converge at a point; this point is known as the focal point of the lens. The first generalization that can be made for the refraction of light by a double convex lens is as follows:

Now suppose that the rays of light are traveling through the focal point on the way to the lens. These rays of light will refract when they enter the lens and refract when they leave the lens. As the light rays enter into the more dense lens material, they refract towards the normal; and as they exit into the less dense air, they refract away from the normal. These specific rays will exit the lens traveling parallel to the principal axis.

The above diagram shows the behavior of two incident rays traveling through the focal point on the way to the lens. Note that the two rays refract parallel to the principal axis. A second generalization for the refraction of light by a double convex lens can be added to the first generalization.

The Thin Lens Approximation

Rules of refraction for diverging lenses.

Now let's investigate the refraction of light by double concave lens. Suppose that several rays of light approach the lens; and suppose that these rays of light are traveling parallel to the principal axis . Upon reaching the front face of the lens, each ray of light will refract towards the normal to the surface. At this boundary, the light ray is passing from air into a more dense medium (usually plastic or glass). Since the light ray is passing from a medium in which it travels relatively fast (less optically dense ) into a medium in which it travels relatively slow (more optically dense ), it will bend towards the normal line. This is the FST principle of refraction . This is shown for two incident rays on the diagram below. Once the light ray refracts across the boundary and enters the lens, it travels in a straight line until it reaches the back face of the lens. At this boundary, each ray of light will refract away from the normal to the surface. Since the light ray is passing from a medium in which it travels relatively slow (more optically dense ) to a medium in which it travels fast (less optically dense ), it will bend away from the normal line. This is the SFA principle of refraction . These principles of refraction are identical to what was observed for the double convex lens above .

The first generalization can now be made for the refraction of light by a double concave lens:  

  Now suppose that the rays of light are traveling towards the focal point on the way to the lens. Because of the negative focal length for double concave lenses, the light rays will head towards the focal point on the opposite side of the lens. These rays will actually reach the lens before they reach the focal point. These rays of light will refract when they enter the lens and refract when they leave the lens. As the light rays enter into the more dense lens material, they refract towards the normal; and as they exit into the less dense air, they refract away from the normal. These specific rays will exit the lens traveling parallel to the principal axis.

The above diagram shows the behavior of two incident rays traveling towards the focal point on the way to the lens. Note that the two rays refract parallel to the principal axis. A second generalization for the refraction of light by a double concave lens can be added to the first generalization.

A Third Rule of Refraction for Lenses

The above discussion focuses on the manner in which converging and diverging lenses refract incident rays that are traveling parallel to the principal axis or are traveling through (or towards) the focal point. But these are not the only two possible incident rays. There are a multitude of incident rays that strike the lens and refract in a variety of ways. Yet, there are three specific rays that behave in a very predictable manner. The third ray that we will investigate is the ray that passes through the precise center of the lens - through the point where the principal axis and the vertical axis intersect. This ray will refract as it enters and refract as it exits the lens, but the net effect of this dual refraction is that the path of the light ray is not changed. For a thin lens , the refracted ray is traveling in the same direction as the incident ray and is approximately in line with it. The behavior of this third incident ray is depicted in the diagram below.

Now we have three incident rays whose refractive behavior is easily predicted. These three rays lead to our three rules of refraction for converging and diverging lenses. These three rules are summarized below.

These three rules of refraction for converging and diverging lenses will be applied through the remainder of this lesson. The rules merely describe the behavior of three specific incident rays. While there is a multitude of light rays being captured and refracted by a lens, only two rays are needed in order to determine the image location. So as we proceed with this lesson, pick your favorite two rules (usually, the ones that are easiest to remember) and apply them to the construction of ray diagrams and the determination of the image location and characteristics.

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light travel diagram

  • The Anatomy of the Eye
  • Light Travels In a Straight Line

Light travels in a straight line can be observed by keeping an object in the path of light. In an atmosphere which is bit dusty, we can see light traveling in a straight line. Light emerging from the torch, train and lamps always travel in a straight line. Let us study in detail how does light travel in a straight line.

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Light travels along a straight line.

Life without light would have been pretty dull. Light travels at a speed of 186,000 miles per second. You must have observed that in your house that whenever a beam of light enters a dark room through a tiny hole in the window, the lightwave always travels in a straight line.

Let us carry out a small activity to show that lightwave travels along a straight line. Take three CD’s and align them together. Align them in such a way that all the CD’s line in a straight line. Now take a candle and place it at the other end. Do make sure that the tip of the candle and the holes of the CD’s all lie in the straight line. Ensure that the height of the CD’s and the tip of the candle are same. Observe the flame of the candle. We are able to see the flame of the candle because the light wave travels through the holes and reaches our eye.

Browse more Topics under Light

  • Reflection of Light
  • Sunlight – White or Coloured
  • Images Formed By Lenses

Now if suppose we displace the center of the CD’s we observe that we are not able to see the flame of the candle. Why does that happen? This is because the light gets blocked. If the light could have the ability to take a curve and travel, we could have seen the lightwave. But since light travels in a straight line, we were unable to see the flame of the candle when the CD is displaced. This proves that light travels along a straight line.

Light travels in a straight line

(Source: Wikipedia)

In the above picture, we can clearly see that light coming through the holes in the window travel along a straight line.

Questions For You

Q1. The phenomenon in which the moon’s shadow falls on earth,  or the earth casts its shadow on the moon, is known as

  • Lateral deviation

Answer: C. The phenomenon in which the moon’s shadow falls on earth or the earth casts its shadow on the moon is known as an eclipse. During a solar eclipse, moon’s shadow falls on the earth. During a lunar eclipse, earth’s shadow falls on the moon.

Q2. Two examples of non-luminous objects are

  • Stars and Moon
  • Burning candle, glowing bulb
  • The moon, a spoon
  • Stars, a spoon

Answer: C. Non-luminous objects are those that do not emit light. The moon and the spoon do not emit light. So these two are good examples of non-luminous objects.

Q3. We can see the objects only when

  • Reflected light from the object reaches our eye.
  • The objects absorb all the light.
  • When the objects allow all the light to pass through them.
  • None of these.

Answer: A. Objects can only be seen when light falls on the object and are reflected back to our eyes.

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  • 3 index cards
  • small piece of modeling clay or sticky tack
  • hole puncher
  • science journal
  • For each index card, use a ruler to draw lines connecting opposite corners of the card.
  • At the intersection of the two lines, use a hole puncher to punch a hole in the center of the index cards.
  • For each card, use a small piece of modeling clay and place the card into the clay to create a "stand" for the card. Place the cards so that they stand vertically and at an equal distance from each other. See Diagram.
  • Place the flashlight at one end of the row of index cards and turn off the light in the room.
  • Arrange the index cards so that light can be seen through all the holes.
  • Observe and record your observations.
  • How can light be seen through all the index cards?
  • What does the experiment prove about the path light travels?
  • What would happen if the holes were smaller?

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  • Light Travels in Straight Line

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An Introduction

Light is one form of energy that plays a vital role in our life. We cannot imagine a world full of darkness. Light makes our vision possible and enhances the beauty of everything around us. Light is playing an important role in both art and science. Light is one of the important tools in science that helps scientists to observe things around the world.

Some theories of science are saying it is particles and some of them are saying light is a wave . If the light is a wave, how does light travel and what is the medium of propagation? Light travels in a straight line. The straight-line path of light is very much evident when light travels through a dusty atmosphere. In this article, we will be discussing the straight line motion of light.

How does Light Travel?

Light can travel through both in a medium and in a vacuum . But in a vacuum, there will not be any particles light can not reflect by hitting it. Hence, in a vacuum, light is invisible. In air, light can be reflected by hitting dust or some other particles, hence light is visible in the air.

Light can be considered as waves. Light waves travel in different wavelengths and depending on the wavelength, different light has different colours. For example, the high wavelength light in visible light has a red colour and the shortest wavelength of light has a violet colour. Being a wave light can show properties of waves such as interference and diffraction .

The answer to the question of how light normally travels is that light travels in a straight line. But the actual answer is light seems to travel in a straight line because of the smaller diffraction effect of light. Diffraction is the bending of waves around an object such that it spreads out and illuminates an area where a shadow is expected.

For light, the wavelength is in the order of nanometers. This wavelength is too small and obstacles of this size cannot be determined by our naked eyes. Hence, we feel that light travels along a straight line. The straight-line motion of light is also called rectilinear propagation of light .

Experiment for the Straight Line Motion of Light

Since the diffraction effect of light is too small, normally light travels along a straight line. By using a simple experimental setup, we can prove that light travels along a straight line.

Place three cardboard sheets back to back in front of a candle on the tabletop. Make sure that the cardboard sheets and the candles are placed in a straight line. Light the candle and make a pinhole on each cardboard sheet. The holes should be made at equal height such that the flame of the candle is visible through them. Now look through the holes and observe light travels in which line. The light flame will be visible along the straight line of holes. Now move one of the cardboard sheets to either side and observe the flame. Can you see the flame? On moving the cardboard sheet, the flame will not be visible. Now, again place the cardboard sheet back in its position. The flame is visible now.

From this experiment, we can conclude that light travels along a straight line and this experiment diagram is given below.

Light travels along a straight line

Examples of Straight Line Motion of Light

Light travels in straight line examples are as follows:.

Light comes out from a torch or train or lamp follows a straight line path.

A straight line path of light is visible when Sunlight comes out through the small holes in a dusty atmosphere.

When we place any opaque object in front of the object, we observe that the object will be invisible. It is because light cannot bend through the corners of the opaque object.

Interesting Facts

Sunlight can reach a depth of 80m in the ocean.

Paul Dirac proposed a theory in explaining the dual nature of light in 1927.

Particles of light are called photons.

The scientist Euclid Catoptrics in 280 BC found light travels in straight-line inhomogeneous media.

Key Features

Light travels along a straight line.

The straight-line motion of light is due to its small diffraction effects.

Light comes out from the train, torch, and lamp are examples of straight line motion of light.

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FAQs on Light Travels in Straight Line

1. What is rectilinear propagation of light?

Light travels along a straight line. The straight-line motion of light is called rectilinear propagation of light.

2. Explain why light travels in a straight line?

Light is a wave exhibiting the property of diffraction. The phenomenon of diffraction is observed only if the wavelength of the wave matches the size of the particle it collides with. Light has wavelengths in the order of nanometers. Usually, an object of nanometer size can not be seen by the naked eye. Hence, the diffraction effect of light is too small to be considered. So, light appears to travel along a straight line.

3. Why is light invisible in a vacuum?

Light can travel through a vacuum. Since in a vacuum there are no particles, light can not reflect. Hence, light is invisible in a vacuum.

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Time Travel: Observing Cosmic History

By observing light from faraway cosmic objects, the Hubble Space Telescope is like a time machine. Light takes time to reach Hubble, because it travels great distances. That means images captured by Hubble today, show what the objects looked like years ago!

A collection of galaxies. On the right side a large spiral galaxy with swirling, twisted arms is flanked by a smaller, but still detailed, spiral behind its arm on the left, and a smaller spiral above it. On the left side is a fourth, round spiral galaxy seen face-on. Between them lies a single bright star. Several stars and distant galaxies dot the background.

The Hubble Space Telescope is many things. It’s an observatory, a satellite, and an icon of cultural and scientific significance – but perhaps most interestingly, Hubble is also a time machine.

Hubble isn’t that far away, locked in a low-Earth orbit just a few hundred miles up that takes about 90 minutes to complete. But with its position just above Earth’s murky atmosphere, Hubble’s transformative view of our universe literally lets us witness our universe’s past.  It allows us to effectively travel back in time.

How does that work? After all, Hubble doesn’t travel beyond our solar system, or even our home planet’s gravity. It certainly doesn’t have any sci-fi elements you might find in Doctor Who or Back to the Future.

Photograph of Hubble orbiting the Earth

Light Travel

The answer is simply light.

The term “light-year” shows up a lot in astronomy. This is a measure of distance that means exactly what it says – the distance that light travels in one year. Given that the speed of light is 186,000 miles (299,000 kilometers) per second, light can cover some serious ground over the course of 365 days. To be precise, almost 6 trillion miles (9.5 trillion kilometers)!

Traveling Back in Time: 8 minutes

Hubble works by gathering light from objects in our universe – some as close as our Moon, and some as distant as galaxy clusters that are billions of light-years away. All that light takes time to reach the telescope, just as it takes time for light to travel from its source to our eyes. For example, our Sun is located about 93 million miles (150 million kilometers) from Earth. That means that it takes roughly eight minutes for its light to reach us here on our planet, so when we look at the Sun (though directly is never recommended!) we see it exactly as it was eight minutes in the past.

Cosmically speaking, the 93 million miles between us and the Sun are nothing. We orbit around just one of billions of stars in the Milky Way Galaxy, which is one of countless trillions of galaxies in the universe.

With that in mind, time travel gets more intense when Hubble observes objects beyond our star system.

Traveling Back in Time: 4 years

Aside from our Sun, the next closest star to us is named Proxima Centauri. It’s about four light-years away, which makes it a close neighbor on a universal scale. But even with Hubble’s sharp, powerful vision, Proxima Centauri remains a point-like object – demonstrating our universe’s unfathomably large size.

Brilliant blue-white star with x-shaped lens flare

Traveling Back in Time: 700 years

Another stellar target of Hubble’s is named Betelgeuse, which is about 700 light-years from Earth. Again, this means that when Hubble looks at Betelgeuse, the star appears exactly as it was 700 years ago. As one of the brightest stars in our sky, astronomers believe it’s likely that even the earliest humans knew of it, as this star appears in stories from several cultures.

This red supergiant star began to dim significantly in the fall of 2019, losing about 60% of its brightness within months. But by April 2020, its regular brightness returned. Hubble studied Betelgeuse and found out that the star “blew its top” – it went through a surface mass ejection, in which the star spewed out a large amount of its surface material into space. When that material in space cooled down, it became a dust cloud that temporarily blocked some of the star’s light.

Hubble’s unique ability to observe in ultraviolet light helped reveal the details of this dimming event and its aftermath. In this range of light, Hubble can better observe the hot layers of atmosphere above a star’s surface.

The telescope continues to be the go-to observatory for scientists who study Betelgeuse. Because it’s taken this long for the light from Betelgeuse to reach us, only in very recent history have we witnessed a cosmic event unfolding that really occurred about 700 years ago!

Scientists also believe that Betelgeuse is on the verge of going supernova – dying in an explosive event. In fact, it may have already done so, but the light from the explosion still hasn’t reached us. There’s a good chance that Betelgeuse no longer exists, though we can still see it today from Earth.

four illustrations of a red-hued star expelling gas, bringing the star into slight shadow

Traveling Back in Time: 6,500 years

Nebulae are clouds of gas and dust where stars are birthed, or the remnants of a dead or dying star itself. These beautiful, ethereal cosmic objects are the subject of some of Hubble’s most iconic images, but they can also teach us more about how our universe behaves and evolves.

For example, a favorite target for Hubble is the Crab Nebula, located about 6,500 light-years away. There are records from 1054 CE written by Chinese astronomers noting the new presence of a shockingly bright “guest” star in the sky, visible even during the daytime. Turns out, they actually saw a supernova – a star’s explosive death – which became the Crab Nebula, made up by the remnants of this violent event. Of course, those Chinese astronomers witnessed a supernova explosion that occurred about 5446 BCE, but it took the light from the explosion 6,500 years to reach Earth in the year 1054.

Bright green, orange, and yellow tendrils intertwined within this egg shaped nebula.

Traveling Back in Time: 2.5 million years

When Hubble looks beyond our own galaxy, we can watch cosmic history unfold over eons.

The Andromeda Galaxy is a whopping 2.5 million light-years away, but that’s just the closest major galaxy to us here in the Milky Way. Observing Andromeda is like staring into a vision from 2.5 million years ago – back during the Paleolithic period on Earth, when very early humans existed.

And if Andromeda is the closest major galaxy to us, it’s difficult to comprehend just how far the light from the most distant observed galaxies has traveled.

This sweeping bird's-eye view of a portion of the Andromeda galaxy (M31) shows stars, lanes of dark dust and bright core. The central region is on the left.

Traveling Back in Time: 12.9 billion years

Another Hubble record is for observing the most distant individual star ever detected, named Earendel. This faraway star emitted its light within the first billion years of the universe, which is about 13.8 billion years old, so it took quite a while to reach Hubble! In fact, that observation was only made possible by nature’s magnifying glass – an astronomical phenomenon known as gravitational lensing. When a massive cosmic object has enough gravity, its gravitational field can magnify and bend light coming from objects located behind it. The gravity of a galaxy cluster located between Hubble and Earendel magnified the star’s light, making it detectable.  The type of star that Earendel seems to be typically have brief lives, only surviving about half a billion years. That means Earendel has ceased to exist for over 12 billion years, yet we are able to look back in time and watch Earendel during its short life.

This Hubble image includes the star Earendel, which is the farthest individual star ever detected.

Traveling Back in Time: 13.4 billion years

Perhaps some of Hubble’s most legendary observations are its deep field images, which collect light from thousands of galaxies that are billions of light-years away.

Field is filled with galaxies in colors of white, yellow, blue-white, and red; all on a black background.

With this type of imagery, we can better understand how our universe changes over time by puzzling out how galaxies evolve. The farther back we look with Hubble, the closer we get to the the big bang, when the universe began – so the most distant galaxies observed by Hubble often appear to us as the “youngest” ones – giving us a sneak peek at the universe in its infancy. Because these galaxies emitted their light when they were young, we get to witness them in their early stages. These early galaxies often appear simpler and smaller than the grandiose spiral galaxies and merged galaxies we see closer to us in distance, and therefore in time. These young galaxies are actually old galaxies now as they have evolved over the time this light has taken to reach us.

Hubble’s farthest observation is of a galaxy named GN-z11, observed as it was 13.4 billion years in the past! This places it within just 400 million years of the big bang itself.

Hubble survey field containing tens of thousands of galaxies including one seen 13.4 billion years in the past

Watching Our Universe Over Time

Observations of the most distant objects, like GN-z11 and Earendel, give astronomers exciting insight into the environment of our early universe. The light we see literally traveled from all the way back then!

Our universe remains mysterious, mind-bendingly large, and ever-expanding, but by gathering light from near and far – from the recent past to the dawn of the universe itself – Hubble helps answer questions about where we are and how the universe works.

At its core, astronomy is really just archaeology. Cosmic objects give off light, letting us learn more about their lives. It can take a long, long time for light to reach Hubble – just one telescope orbiting just one planet in just one solar system in just one galaxy. Scientists use Hubble like the time machine that it is to piece together the history and mystery of the cosmos, giving us all a glimpse right up to the edge of the universe – and time itself.

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Uncovering How Microscope Light Travels: A Comprehensive Guide

Michael Oliver Barlow

Updated on: 06.03.2023

light travel diagram

If you have ever looked through a microscope, you may have wondered how a microscope light travels to illuminate the specimen you are observing. Understanding the science behind illumination is crucial for obtaining clear and accurate images. In this article, we will explore how a microscope light travels and how it is essential for microscopy. Whether you’re a novice or an experienced user, this knowledge will help you to better understand how to adjust and optimize microscope settings to achieve the best results. So, let’s delve into the fascinating world of microscopy and uncover the secrets of how a microscope light travels.

Types of Light Sources

Types Of Light Sources

Tungsten Bulb

Tungsten bulbs are a type of incandescent bulb that have been used for decades as a light source for microscopes. They work by passing an electric current through a tungsten filament, which heats up and produces light. Tungsten bulbs emit a warm, yellowish light and have a color temperature of around 3200K. However, they are not very energy efficient, and most of the energy that they consume is converted to heat rather than light.

Fluorescent Bulb

Fluorescent bulbs have been used as a light source for microscopes since the 1950s. They work by passing an electric current through a gas, which emits ultraviolet radiation that is then converted into visible light by a coating of phosphor on the inside of the bulb. Fluorescent bulbs are more energy-efficient than tungsten bulbs, and they usually have a color temperature of around 6500K, which is similar to daylight.

LED bulbs are a more recent development in microscopy lighting. They work by passing an electric current through a semiconductor material, which emits light. LED bulbs are extremely energy-efficient, and they have a much longer lifespan than tungsten or fluorescent bulbs. They also emit a more natural, white light that is similar to daylight. LED bulbs can be adjusted to produce different color temperatures, which is useful when examining specimens under different lighting conditions.

How does light travel through a microscope to your eye? Light from the microscope’s light source passes through the condenser lens and through the specimen. The light that passes through the specimen is then magnified by the objective lens and focused onto the eyepiece. The eyepiece then further magnifies the image and projects it onto the retina of your eye.

How Does Light Travel Through a Microscope?

How Does Light Travel Through A Microscope?

One way that light travels through a microscope is through reflection. This occurs when light waves bounce off the surface of an object, following the law of reflection. The angle of incidence of the light wave is equal to the angle of reflection.

Microscope mirrors and lenses use reflection to redirect and focus light onto the specimen being observed. In some cases, multiple mirrors and lenses are used to increase the magnification of the image.

Another way that light travels through a microscope is through refraction. This occurs when light passes through a medium with a different refractive index, causing the light to bend. The amount of bending depends on the angle at which the light enters the medium and the refractive indices of the two media.

Microscope lenses use refraction to bend and focus light onto the specimen. Different types of lenses, such as converging and diverging lenses, are used to create various magnifications and resolutions.

Diffraction

Diffraction is another way that light travels through a microscope. This occurs when light waves encounter an obstacle or aperture that is similar in size to the wavelength of the light. The light waves spread out and interfere with each other, creating a diffraction pattern.

Microscope users can take advantage of diffraction to create high-resolution images. By using a smaller aperture or pinhole, the diffraction patterns become more pronounced and allow for better resolution of the specimen being observed.

Illumination Systems

Illumination Systems

Reflective Illumination

Reflective illumination is a type of illumination system used in microscopy that involves directing a beam of light onto the specimen. This light is reflected off of the surface and into the objective lens of the microscope, which then magnifies the image.

This type of illumination is commonly used in bright-field microscopy and is essential for observing specimens that are not capable of transmitting light, such as opaque samples.

The advantages of reflective illumination are:

  • Allows for observation of small or opaque samples
  • Provides strong contrast
  • Produced no glare
  • Easier to set up than other illumination methods

Transmitted Illumination

Transmitted illumination is a type of illumination system that involves directing light through a thin section of the specimen. This type of illumination is commonly used in bright-field microscopy as well, but is also utilized in phase-contrast microscopy.

A light source located under the microscope stage shines light through the sample, and the transmitted light is then magnified by the objective lens.

The advantages of transmitted illumination are:

  • Gives a clear view of thin, transparent samples
  • Allows for contrast to be added to the specimen in various ways
  • Used to visualize fluorescence, phase contrast, and DIC specimens

Understanding how light travels through a microscope allows us to see the many different factors that can affect our observations and helps us to make informed decisions regarding the use of illumination systems.

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Objectives are lenses that are located near the slide under examination, and they form the initial magnified image of the specimen.

These lenses have a strong effect on the quality of the final image produced. High-quality objectives are designed to avoid any distortion, spherical or chromatic aberrations, that can hinder an accurate image.

Eyepieces, also known as ocular lenses, are situated on the microscope’s upper part and are where the observer looks through.

They receive light from the objectives and magnify the image even further.

High-quality eyepieces are intended to deliver a clear, extended, bright, and detailed image for the observer.

The condenser system on a microscope is located beneath the stage and serves as the focal point to concentrate light onto the specimen.

It ensures uniform illumination of the entire field of view and plays a critical role in light control by regulating the amount of light reaching the specimen.

High-quality condensers can produce a much brighter, sharper, and higher resolution image by using an advanced condenser optical design with adjustable aperture and contrast improvement techniques.

The diaphragm is a mechanism within the condenser that controls the amount of light that hits the specimen.

It controls the cone angle of the illuminating light ray and helps to regulate the depth of field and contrast of the specimen image.

High-quality diaphragms allow consistent and uniform lighting, along with the ability to alter the level of illumination according to specimen type and required amplification.

Chromatic Aberrations

Chromatic Aberrations

One of the main challenges in illumination systems of microscopes is the problem of chromatic aberrations. Chromatic aberration is a phenomenon that causes colors to appear differently when viewed through a lens or optical system. In simple terms, it is the inability of lenses to focus different wavelengths of light at the same point, resulting in a blurred or colored image. Chromatic aberrations can cause the edges of an image to appear blurry, and can also result in a halo effect around objects.

Chromatic aberrations occur because the refractive index of a lens material varies with the wavelength of light passing through it. This means that different wavelengths of light are bent or refracted by different amounts as they pass through the lens, causing them to focus at slightly different points.

There are different types of chromatic aberrations, including longitudinal chromatic aberration and lateral chromatic aberration. Longitudinal chromatic aberration causes different colors to be focused at different distances from the lens, resulting in a blurred image. Lateral chromatic aberration affects the position of the image for different colors, causing the image to appear shifted or distorted.

To correct for these chromatic aberrations, researchers have developed different types of lenses and coatings. Achromatic lenses, for example, are designed to reduce chromatic aberration by combining multiple lenses made of different materials. Apochromatic lenses are another type of lens that corrects for both chromatic and spherical aberrations.

In addition to lenses and coatings, researchers have also developed computational methods for correcting chromatic aberrations in images. These methods involve using software algorithms to analyze and correct for the color differences in the image.

Overall, understanding the science behind chromatic aberrations is crucial for developing effective illumination systems for microscopes. By understanding these phenomena and developing appropriate solutions, researchers can improve the quality and accuracy of microscope images, enabling new discoveries and advancements in science and technology.

Polarization

Polarization

Polarization is an important concept in microscopy illumination. It is the property of light waves that refers to the orientation of the electric field in the waveform. The orientation of the light wave is either vertical, horizontal, or randomly oriented.

When light travels through a microscope, sometimes it may become polarized by interacting with the sample or optical components of the microscope. There are different types of polarization, including linear, circular, and elliptical.

Linear polarization is when the orientation of the electric field in the light wave is in one direction. This can be achieved by passing light through a polarizer, which only allows light waves with a certain orientation to pass through.

Circular polarization is when the electric field rotates around the axis of the light wave. This type of polarization is generated by using a quarter-wave plate or a circular polarizer.

Elliptical polarization is when the electric field rotates elliptically around the axis of the light wave. This type of polarization can be created by using a combination of polarizing elements.

Polarization can be useful in microscopy as it can improve image contrast and reduce glare or unwanted reflections. It can also be used to study the optical properties of materials or to identify crystal structures.

In summary, polarization is an important property of light waves in microscopy. Understanding the different types of polarization and their applications can help improve imaging techniques and scientific studies.

Fluorescence Microscopy

Fluorescence microscopy is a powerful technique that enables scientists to visualize and study biological samples at the cellular level. It is based on the phenomenon of fluorescence, which occurs when certain molecules absorb light of a specific wavelength and then emit light at a longer wavelength.

In a typical fluorescence microscopy setup, a light source such as a mercury or xenon arc lamp is used to illuminate the sample with a specific wavelength of light. The sample is often treated with a fluorescent dye or protein, which selectively binds to certain molecules of interest and emits light when excited by the light source.

The emitted fluorescent light is then captured by a specialized microscope objective, which focuses the light onto a detector such as a camera or photomultiplier tube. The detector converts the light into an electrical signal that can be processed and analyzed by a computer.

One of the key advantages of fluorescence microscopy is its ability to selectively label specific molecules in a sample, allowing researchers to visualize their distribution and movements within cells. For example, fluorescently-labeled antibodies can be used to detect the presence of specific proteins in a sample, while fluorescently-labeled nucleic acid probes can be used to detect specific DNA or RNA sequences.

In addition, fluorescence microscopy can be combined with other techniques such as confocal microscopy, which uses a pinhole to eliminate the out-of-focus light and improve image resolution, and two-photon microscopy, which uses two photons of lower energy to excite the fluorescent molecules and reduce phototoxicity and photobleaching.

Despite its many advantages, fluorescence microscopy also has some limitations. For example, the fluorescent dyes or proteins can sometimes interfere with the normal function of the molecules being studied, and the fluorescence signals can be affected by factors such as photobleaching and quenching.

Nevertheless, fluorescence microscopy continues to be a critical tool in the field of biological research, allowing scientists to unlock the secrets of the microscopic world and advance our understanding of the fundamental processes of life.

Frequently Asked Questions

What types of microscopes require illumination.

  • Stereo microscopes:

Stereo microscopes, also known as dissection or low power microscopes, are used to observe specimens at a relatively low magnification range of up to 200x. These microscopes require illumination to produce bright and clear images of the specimens under observation.

  • Compound microscopes:

Compound microscopes are high power microscopes that are used for more detailed image observation in the range of 40x to 1000x magnification. These microscopes require proper illumination to visualize the small and intricate details of specimens.

  • Fluorescence microscopes:

Fluorescence microscopes are used to observe living cells, proteins, bacteria, and viruses. They use a special type of illumination that excites fluorophores within the specimen, causing them to emit light. This makes them easily visible and distinguishable from the surrounding tissue. Therefore, fluorescence microscopes require highly sensitive illumination systems.

  • Polarizing microscopes:

Polarizing microscopes are used for the observation of minerals, crystals, and other anisotropic specimens that require polarized light to visualize their properties. They require high-quality lighting to produce the necessary polarized light.

  • Darkfield microscopes:

Darkfield microscopes are specially designed for the observation of highly transparent specimens that are difficult to see under normal bright-field illumination systems. They require darkfield illumination to produce high contrast images.

In conclusion, different types of microscopes have different illuminating requirements based on their functions and capabilities. It is important to have proper illumination in microscopes to achieve better image quality and clarity.

How does the light source affect the image quality of a microscope?

The light source plays a crucial role in determining the quality of the image that is produced by a microscope. Here are some ways in which the light source can affect image quality:

  • Brightness: The brightness of the light will determine how well you are able to see the image. If the light is too dim, the image will be difficult to see, while if it is too bright, it may wash out details.
  • Color: The color of the light can also affect the image quality. Different wavelengths of light can cause certain areas of the specimen to appear differently, which can be useful in certain applications.
  • Uniformity: If the light source is unevenly distributed, this can cause certain parts of the image to be over or underexposed, making it more difficult to accurately interpret the specimen.
  • Directionality: The direction from which the light shines can also have an impact on the image quality. If the light is oblique, it can bring out certain features of the specimen, while if it is direct, it may cause glare or reflections that can make the image more difficult to see.

Ultimately, the choice of light source will depend on the specific application and the nature of the specimen being observed. However, it is important to keep in mind how different lighting conditions can affect the image quality, and to adjust the lighting as necessary to achieve the best possible results.

How does the light travel through the microscope?

When light travels through a microscope, it enters through the source and passes through the condenser lens. The condenser lens focuses the light onto the specimen, which then scatters the light in all directions. Some of the light is reflected back through the objective lens and is further magnified. The remaining light passes through the objective lens and ultimately reaches the eyepiece, where it is further magnified and focused onto the viewer’s eye.

It is important that the microscope is properly illuminated to allow for successful viewing of the specimen. Adjusting the intensity and direction of the light source can also affect the clarity and contrast of the image. Understanding how light travels through a microscope can help in achieving optimal illumination and improved viewing of the specimen.

What is the difference between transmitted light and reflected light microscopy?

When it comes to microscopy, understanding how light travels is crucial. There are two primary methods of illumination in microscopy: transmitted light and reflected light. Let’s take a closer look at the differences between the two.

  • Transmitted light microscopy: This method involves light passing through the specimen and into the objective lens. This illumination technique is most commonly used with transparent or thin specimens, such as cells, tissue sections, or histology samples. A significant advantage of transmitted light microscopy is the higher level of resolution it provides, making it ideal for detecting small details.
  • Reflected light microscopy: In this method, the light beam is directed onto the surface of the specimen, and the reflected light is then collected by the objective lens. This technique works well for opaque samples, such as metals or ceramics. Due to the nature of reflected light, it is not typically used for examining the internal structures of specimens, but rather for studying their surface features.

Additionally, there are two common types of illumination techniques used in microscopy: brightfield and darkfield. In brightfield microscopy, the specimen appears dark against a bright background, while in darkfield microscopy, the background is dark, and the specimen appears bright. Both of these techniques can be used with either transmitted or reflected light illumination.

Overall, understanding the differences between transmitted and reflected light microscopy can help inform which method is best suited for a particular sample or application.

What safety precautions should be taken when using a microscope light source?

When working with a microscope light source, it is important to take certain safety precautions to ensure that you do not cause any harm to yourself or others. Firstly, never look directly into the light source or point it at anyone’s face as it can cause eye damage. Always switch off the light source before changing bulbs or making any adjustments to prevent electrical shocks. In addition, regularly clean the light source and the microscope lens to avoid any dust or debris that may obstruct the view. Finally, if working with a halogen lamp, never touch the bulb with bare hands as the oils from the skin can cause it to crack or even explode. Always use protective gloves or a clean tissue while handling the bulb. By following these safety guidelines, you can ensure a safe and successful experiment with your microscope light source.

Microscope illumination works by illuminating the sample with a beam of light and then focusing the light onto the sample. The light travels through the microscope objective, is absorbed by the sample, then is reflected back through the microscope objective and finally projected onto the eyepiece for the viewer to observe. By understanding how this works, it is possible to optimize microscope illumination for the best results.

  • 1. Chang, M. (2010). Microscope optics. Methods in cell biology, 97, 1-28.
  • 2. Asghar, A. & Wu, B. (2009). Numerical investigation of light intensity distribution in a microscope system. Optics Express, 17(21), 10799-10812.
  • 3. Singh, M., & Akers, W. (2015). Illumination in microscopy: The effect of lamp, magnification and numerical aperture on light intensity. Journal of Visualized Experiments, (106).

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  22. Refraction of a Wave of Light

    In physics and optics, a medium refers to any material through which light or other electromagnetic waves can travel. It's essentially a substance that acts as a carrier for these waves.; Light is a form of electromagnetic radiation, which travels in the form of waves.These waves consist of oscillating electric and magnetic fields.; The properties of the medium, such as its density and ...

  23. Time Travel: Observing Cosmic History

    Light Travel The answer is simply light. The term "light-year" shows up a lot in astronomy. This is a measure of distance that means exactly what it says - the distance that light travels in one year. Given that the speed of light is 186,000 miles (299,000 kilometers) per second, light can cover some serious […]

  24. Uncovering How Microscope Light Travels: A Comprehensive Guide

    Reflection. One way that light travels through a microscope is through reflection. This occurs when light waves bounce off the surface of an object, following the law of reflection. The angle of incidence of the light wave is equal to the angle of reflection. Microscope mirrors and lenses use reflection to redirect and focus light onto the ...