Q: If light slows down in different materials, then how can it be a universal speed?

Physicist: This bothers a lot of people.  When you’re learning physics, there are several things that you learn in the first couple years.  Among them are:

1) The speed of light is an absolute.

2) Light slows down when it passes through a medium (like water, glass, air, …).

The first statement is the backbone of all of modern physics (relativity), and the second helps explain things like diffraction and rainbows.  But clearly these statements contradict each other.

Here’s the idea: a medium, whatever it is, is made up of molecules.  When a photon (light particle) hits a molecule it is sometimes absorbed.  Its energy is turned into raised electron-energy-levels, or vibrations and flexing, or movement.  In short order (very short order) the photon is spit out of the other side, none the worse for wear.

When a photon hits a molecule it's sometimes absorbed and re-emitted. That process takes a little bit of time that we interpret as a "slowing".

In between molecules light still travels at light speed.  It’s just that, with all those molecules around, it’s always darting ahead, getting absorbed, pausing for a moment, then being re-emitted.  On the scale we’re used too, this happens so much and so fast that you don’t notice the starting-and-stopping.  Instead you notice an average slowing of the light.

That is, if light always takes about 33% longer to travel through water than air (and it does) due to absorption and re-emission, you’d say “ah, light travels slower through water!”.  The fact that that isn’t quite the case is rarely important.


Answer gravy: This isn’t part of the answer, but it’s interesting: The interaction between light and the medium it moves through is generally “clean”, in the sense that even if the light is in a complicated quantum state before entering the medium, it retains it.  As a result, light continues to point in the same direction (which is good, in terms of seeing stuff), and even keeps its polarization.

What’s really fascinating, is that even more bizarre quantum states, like those involving being spread out over a large area, are also allowed to persist.  If this were not the case it would be impossible to do the double slit experiment (which requires the photon to be in many locations) without a specially prepared vacuum chamber!

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50 Responses to Q: If light slows down in different materials, then how can it be a universal speed?

  1. janne says:

    Why is it that the interactions between photons and the propagation medium are so clean?

  2. Sanal Kumar MS says:

    “”In between molecules light still travels at light speed. It’s just that, with all those molecules around, it’s always darting ahead, getting absorbed, pausing for a moment, then being re-emitted.””

    so for a moment the velocity of light become zero???

  3. The Physicist The Physicist says:

    It would be more accurate to say that, for a moment, the light is destroyed.

  4. Matt says:

    janne: The ‘clean’ interaction he described here is a special case for transparent materials. In general the emitted photon can have a different wavelength and direction from the incident one – this is called scattering. The type of scattering that occurs depends on how the incident photon’s energy compares with the electron energy levels in the molecule.

  5. Michael Camiloto says:

    Forgive me if I’m just confusing concepts here, but how is refraction explained in this context? If one “side” of the light hits a new material and starts being absorbed/re-emitted a small period of time before the other “side” impacts, how does that change the angle?

  6. David Minster says:

    Light does not speed up or slow down. The velocity of light is a constant for the medium through which it travels.

  7. kanishk says:

    If speed of light changes in different medium then the kinetic energy(1/2mv^2) possessed by light should also change and it should be liberated in some way like energy or something else but I have never seen something like this happening?

  8. The Physicist The Physicist says:

    Sure, but the point of the post was that the speed of light doesn’t change. The energy changes form, and the light starts and stops, but when it’s moving it’s moving at the usual speed.

  9. galaxy says:

    Why is the speed of light finite? and why is it at the current speed? why not slower? or faster?

  10. The Physicist The Physicist says:

    Q: Why is the speed of light finite?

    As for why it is what it is? Who knows. You can make an appeal to the anthropic principle, but that almost certainly doesn’t explain the values of all constants.

  11. MIKE says:

    light being absolute as in the speed of light , and i think but not sure as with falling in our atmasphere we have terminal velocity in space does mater have its own terminal velocity as light traveling through space has an maxium speed light speed but space is made up of matter and considering the big bang we are in a expanding universe so on the outskirts of our universe light and matter would travel faster ?

  12. The Physicist The Physicist says:

    As far as we can tell, the universe doesn’t have “outskirts”. Every point is more or less the same as any other point.
    As for light, it also behave the same way regardless of where it is: always moving at light speed.

  13. DS says:

    The problem with this explanation is that it predicts that the time it takes light to traverse the glass should be directly proportional to the thickness of the glass. In fact, this is not the case–the speed of light through a particular type of glass is always a constant.

  14. The Physicist The Physicist says:

    Why is it proportional to the thickness of the glass?

  15. theSpleen says:

    It is true that the time it takes to travel through glass is proportional to the thickness. But that implies that the velocity is constant, which is just the case!

  16. Cory says:

    Could space be a medium, being made up of dark matter? And it has a terminal velocity, the same way that matter can only “move air molecules out of the way” at a certain rate here on earth while being pulled by its gravitational pull, light can only do the same with dark matter, thus giving it what could be considered the terminal velocity of light through space?

  17. The Physicist The Physicist says:

    Weirdly enough, the properties of spacetime (as we understand them) are incompatible with the properties of a material. The fact that the speed of light is the maximum speed isn’t due to some kind of property of space, in the way the speed of sound is dictated by the properties of air, it’s due to the fact that no matter how you move, light is always moving at the same speed. There’s an old post here that goes into more detail.

  18. Rod says:

    Physicist. I agree with Cory. Space is filled with subatomic particles. IF Light slows down because of the size of the quantum particles it is traveling through e.g. a water molecule very close to another water molecule would presumably slow down light more in water’s liquid state than in it’s gaseous state where the quantum particles are further apart. Now, given that the subatomic particles traveling through space are even smaller and further apart they slow down light (less than) say steam does. BUT they SLOW down light. Which means in the absence of subatomic particles light would travel much faster than 186K MPS. Maybe even infinitely fast, which then explains how the Big Bang could expand faster than light, no subatomic particles slowing it down. Correct?

  19. Rod says:

    I meant to say in the last post “…….because of the size and proximity of the quantum particles it is traveling through…..” Also wouldn’t this explain why we travel so fast through wormholes theoretically? What if a wormhole is simply just a place in space devoid of quantum particles to slow down light or anything else traveling through the wormhole?

  20. The Physicist The Physicist says:

    The speed of light is more than just a speed that some particular thing (light) moves at, it’s an important fundamental constant. The central pillar of relativity is that the speed of light, in vacuum, is the same for every possible observer (the speed of light through a material is unimportant).
    Something traveling faster than C, for whatever reason, is impossible for a lot of reasons all having to do with relativity.
    Wormholes are described in terms of weirdly twisted up spacetime, as opposed to “particle-free highways”.

  21. Rod says:

    Thanks for the quick response. This site is pretty a cool hangout for a bored aerospace engineer. So…..I’ll bet you that 30 years from now all of the far fetched things that I have proposed will be accepted scientific theory and fact. In all of the speculation (I won’t even insult you by calling it theory) that I have proposed, the one thing that holds it all together is in order to calculate the speed of light through all mediums (and I would propose that subatomic particles in “empty space” are a medium that make up dark matter) the thing that holds it all together is that in order to calculate the speed of light through a medium we must know that medium’s “index of refraction”. What is the index of refraction of dark matter? You CAN’T say zero, yet. AND for c to be light’s speed limit, dark matter would have to have an index of refraction of ZERO. :)

  22. Rod says:

    Actually I retract that, the index of refraction of dark matter would have to be 1 both with or without the presence of sub atomic particles in space. IF the index of refraction of empty space with subatomic particles is one and without subatomic particles it is LESS than 1(where the speed of light would be faster than c) even down to zero(where the speed of light would be infinity). Then my speculation would hold true.

  23. Rod says:

    LOL, you may take my speculation and accept your Nobel prize now. :)

  24. The Physicist The Physicist says:

    The Nobel committee stopped returning my calls years ago.
    You bring up an interesting point, but because dark matter is completely electrically neutral (and thus doesn’t interact through the electromagnetic force) it doesn’t really have an index of refraction. Or, because it doesn’t even enter consideration, its index is 1.

  25. Rod says:

    Good answer, however, my wild speculation in 30 years give or take 10 will prove correct I would suspect. Interesting topic. I loved quantum mechanics in college and theoretical physics. I would have become a physical chemist of I could have afforded hanging around the university longer, but had to leave theory and go applied and become an engineer to make some money. If I’d stayed, you’d be seeing my wild ramblings in Scientific American giving everyone a headache. Give me access to a particle accelerator and all hell breaks loose. :) Thanks all. Great topic.

  26. Anand abhishek says:

    do forces existing between subatomic particles influence the speed of light … ??

  27. Ryan says:

    What causes the light maintain its original direction? I’ve always been told that when an atom emits light it does so in a random direction.

  28. Ibrahim says:

    hey,sorry but my problem here is that the absorption and re-emission occurs for the electrons of the atom not for the bound not for the free electrons now the possibility of the photon to hit the electron is very low and although i clearly see the broken ray ,so i found it hard to take in your explain sir can you explain more please.

  29. Ibrahim says:

    sorry sir but can not agree very more with that as the most of atom is space how can the most of photons be absorbed and re-emitted although most atoms are space ????

  30. tacoman says:

    Does lightspeed lose traction around tight corners or on dirt roads?

  31. Bob says:

    Most of the time, you would say that the speed of light in a vacuum is absolute, at 299,792,458 m/s. Then, you would avoid this entire problem! Right?

  32. tom says:

    I know that the speed of light in a vacuum is always measured to be the same regardless of the observer’s frame of reference. My question is, does the same apply to the speed of light as it travels through any non-vacuum medium?
    Say light travels at 0.9c through water, as measured from a stationary observer relative to the water. Will it travel at 0.9c through the water as measured by an observer moving 0.9c relative to the water?

  33. The Physicist The Physicist says:

    Nope!
    You can even out-run light if it’s traveling through a medium.

  34. John Rolin says:

    Regarding the bending of light as it passes from one medium to another, I can grasp the explanation that a wave front bends. One part of the wave front hits the new medium first and begins to slow due to absorption and re-emission. Whereas, another part of the wave front hits the new medium later. But, what if one produces a single photon beam of light and aims it at a transparent medium at an oblique angle, how does it bend? Or will it be bent? Does the Heisenberg uncertainty principle become involved? Surely the first example is describable using classic physics, whereas, the single photon example must invoke QM.

  35. John Rolin says:

    This topic also brings to mind something I heard when I was an undergraduate in physics. Regarding reflections from a pane of glass, the amount of light reflected back is ~ 4% and that no one can explain why 4%. Has someone been able to explain this phenomena in the last 50 or so years since I was an undergrad? Also, why is there any reflection at all from a transparent substance like glass?

  36. The Physicist The Physicist says:

    You do need to bring in quantum mechanics, but it turns out that it doesn’t change much. In QM you describe individual photons as waves too, and the math all goes through in essentially the same way.

  37. The Physicist The Physicist says:

    Reflection off of transparent media is pretty well understood, but it’s not a one-line answer.
    I’ll do a post soon.

  38. John David Dunson says:

    i’d be very interested in reading that post as well. one of my favorite physics books is Q.E.D. by Feynman. i love QED. would you happen to have any suggestions on good books with that topic?

  39. Bob Beharie says:

    May I point out the the theory of relativity is not called a ‘theory’ for nothing… Look at the definition in your dictionaries. It seems to me there is a lot people wasting time and money trying to force ‘relativity’ to explain what we can observe particularly in regards to the formation and age of the universe.

  40. Osama Bin Imran says:

    So if say the light is moving at a speed of 0.7c through glass, if an observer is running parallel to the glass in the same direction as the light, will the light still have a relative velocity of 0.7c with respect to the observer just like in a vacuum..?

  41. Rudra Upadhyay says:

    hey cud u plz tell me that is there any existence of some gaseous molecule or a particle which absorbs, stores and emit light ????

  42. LdB says:

    I like your attempt at the answer but it is easy to show how wrong it is.

    Atoms only absorb at specific frequencies if it worked as you have said above only very specific frequencies would be slowed … quantum mechanics says that. Air and glass for example slow the entire visible light range of frequencies something that is impossible under your answer.

    There is a second obvious disproof of your answer which would be to violently heat the media so the molecules are jiggling around violently. When the atoms absorb the photon and then re-emit it as per via your answer gravy the light should fly off in skewed directions making the beam fatter if the effect is small or like an old 70’s disco ball if the effect is extreme. Pass a laser pointer thru a boil flask of water if you want to check!

    This problem is a doozy to explain to layman I will be interested to see if you have another go at it.

  43. The Physicist The Physicist says:

    @LdB
    While it is true that light can only be emitted or absorbed at specific wavelengths, most materials are complex enough that the number of allowed transitions is truly massive. Even before taking into account vibrational modes, translational modes (for gases and liquids), rotational modes, etc., there are often already so many electron energy levels available that they have to be described as energy bands where practically any photon (within the correct range of frequencies) can be absorbed. If this weren’t the case, then everything would be (mostly) clear and nothing would be dark-colored.
    You’re right about the temperature being important, but boiling water doesn’t cut it. The molecules in a pot of boiling water are only moving at about 700 m/s. For comparison, light is traveling at about 300000000 m/s. As far as light is concerned, the boiling water is sitting still. By the time a photon has passed into and back out of the pot, the water molecules have only had the chance to move about half a micrometer.
    About the only time you’d see light being grossly moved around by the atoms it briefly inhabits, would be during a stellar collapse or inside of a white dwarf or something (where it’s crazy hot). That said, we do see “blurring” of spectral lines due to heat all the time.

  44. LdB says:

    Again you have missed the point and given your answer relative to a solid. In glass for example there is a lattice structure has properties and you could argue that the lattice absorbs wider frequency ranges and I have seen some scientists make this mistake. The problem is air is not a solid and there is no lattice and yet it acts as a media.

    I will give you another example of the error in the above as you explained it is wrong. If it works as you describe above there would be two other effects you would see.

    1.) The speed of light in the media would get slower the longer the media because you would have more absorptions and thus more delays. Again I have seen people try and use the lattice version to try and get around this problem but air has no lattice it is all a band aid.

    2.) Under the description above there exists the possibility to make an incredibly thin piece of material that is one atom thick and now you have something weird that we have two speeds of light. If the photon hits an atom you get the delayed version and if it doesn’t hit the atom then you don’t. So your only way around the problem is to try and do like you did with air and claim there is a 3D lattice.

    The answer is wrong even to try use as a layman version and no amount of triage can save it as it leads to wrong conclusions.

    I have used your site to refer students to because in general it is a great resource but on this matter it misses wildly.

    The correct answer is given under “slow light” in Wikipedia anything that contains electrons is going to slow light down and even a single atom layer of glass will refract light passing thru it and yes it will slow light down within the 1 atom wide sheet but there never exists the possibility of two speeds of light as per your answer. The Wikipedia article will also explain there are a number of different ways to slow light down and all have experimentally been done.

  45. John David Dunson says:

    @Ryan
    “What causes the light maintain its original direction? I’ve always been told that when an atom emits light it does so in a random direction.”
    Light is emitted and travels in ALL directions simultaneously. The reason it seems to be travelling in a one certain direction is because the waves are out of phase in all others.

    @tacoman
    “Does light speed lose traction around tight corners or on dirt roads?”
    No. Light can curve around corners and maintain its speed.

    @John Rolin
    “Would a single photon bend when travelling between mediums?”
    The light doesn’t actually bend. As I said above, light actually travels in all directions simultaneously. As it travels through a medium, like air, the different waves align and cancel out in a certain pattern, making it seem like it’s travelling in a certain direction. When it transitions into a new medium, the phases are going to align and cancel out at a different rate, essentially making its path appear to bend. So, yes, a single photon would travel in a new direction regardless of whether or not it has any friends along for the ride.

  46. Swaroop says:

    The picture that light moves slower in a medium is because photons get absorbed and re-emitted introduces a random process since the delay is random, although, clearly, refraction is not a random process – If you shine a laser on the side of a glass slab, the final emmitted light is a point also – The interpretation that appeals to me personally, is the quantum mechanical one, although i do not understand it completely. It is that a photon always follows all possible paths through a the medium and it is the superposition of all these paths that produce a resultant slower wave.

  47. RJ says:

    The Electromagnetic spectrum. (Is photon speed constant?)

    If photons always shift at a constant rate, why then do photons transform into a lesser (heat-energy) when it collides into objects?
    Such as visible photons(light) absorbing into a dark(black) surface… Does heat travel at the speed of light too? Hahaha! jk.

    It seems that the photons no longer have the same frequency or energy level as they started with.
    Does that also mean that they maintain the same exact rate as they hit the surface of the material?
    I suppose then, if the photon can escape before it loses all of its energy, it will be able to re-accelerate again to its maximum-limit once again…?

    The wave’s(photon’s) energy level(speed) is always reduced from the collision with the material (from resistance with an obstruction, as in a car hitting a wall).
    So when waves “die”(transforms/ dissipates), it collides with an object and the reaction generates heat from the friction(unabsorbed energy from the transfer).

    Naturally, all heat molds right back into what is was, and goes right back where it came from in the first place, which is from Dark Energy.
    Dark energy is the intense binding force between existence(matter/waves/vibrations) and “The Nothing” that embeds all of the infinitesimal fabric of space(No waves/vibrations).
    When the struggle(interaction/resistance/opposition) between existence(intersecting waves/vibrations) and “The Nothing” occurs, heat is generated.
    Uhhhhmm, I think.. Yaaaawwwwwwwwnnnnnnn, I really need sleep now…

  48. Tommy Klepac says:

    What force or forces act upon a photon to start and move at such a great speed and does it “speed up” when nearing the gravitational pull of a much larger object? It is, after all,
    a partical with mass.

  49. The Physicist The Physicist says:

    @Tommy Klepac
    Photons are bizarre things. They exist at the speed of light; never accelerating or slowing down under any circumstances. There’s no force that gets them moving. In fact, photons are the force carrier for the electromagnetic force (they are what conveys that force between other particles).
    They are influenced by gravitational fields, but no so much because they have mass (they don’t) but because everything is always influenced by gravity.

  50. Kendall Roberson says:

    I’m sorry, but this answer is just wrong. If the speed of light in a medium were due to absorption and re-emission of light, you would get some “lucky” photons that make it through quickly without hitting any atoms and some “unlucky” photons that hit a lot of atoms and are delayed. In that case a transparent medium would spread the speed of light out proportional to molar volume and atomic cross section rather than uniformly change the speed of light.

    I’ve researched this question across the web several times, and run into a lot of people just saying essentially “it changes the speed of light because it does” with a lot of words obscuring their ignorance OR I run into this absorption / re-emission explanation which doesn’t fit actual behavior…. Example: What if you shine a particular wavelength of light through a pure medium that doesn’t have any electron orbitals available to absorb that particular wavelength of light? Based on your absorption / re-emission theory that light in that case would travel at the speed of c in a vacuum, but that is not what is observed. Additionally it is unclear how an atom would ‘know’ to diffuse the photon and re-emit it in a random direction vs. when it should re-emit it in its identical original direction.

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