Some places I've read flat out say light actually slows down in a medium, some say the speed decresae is just apparent but not real and that the photons still travel at the constant speed of light.

I'm kinda confused.

  • $\begingroup$ As with many other questions in E&M you must be clear what framework you are using. The first part of your title implies the QFT picture, for which there is a clear and simple answer about photon behavior. But the second part or your title requires relating that picture to the classical approach and that is non-trivial as it requires an explanation of coherent forward scattering (which is not a subject that evey graduate student meets in any detail). $\endgroup$ Jan 13, 2020 at 18:06
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    $\begingroup$ Possible duplicates: physics.stackexchange.com/q/466 physics.stackexchange.com/q/11820 $\endgroup$ Jan 13, 2020 at 18:18
  • $\begingroup$ This question popped to mind due to a silly discussion I had over fiction. One character can launch light based attacks and turn into photons, so the question raised is: do the air molecules in his way decrease his speed by a small margin or does he travel at top speed by default? Just to be clear: I've got no real in-depth knowledge of physics. English is also not my first language, so I might have hard time understanding some explanations. In fact, we weren't even taught physics at school(go figure). $\endgroup$
    – BobiX
    Jan 13, 2020 at 18:26
  • $\begingroup$ There is a real difference of speed. See Fizeau-Foucault apparatus at wikipedia. $\endgroup$ Jan 13, 2020 at 22:53
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    $\begingroup$ The photons themselves travel at the exact same speed of $c$ but the path they end up taking in an optically denser medium is much longer. $\endgroup$
    – Sam
    Jan 14, 2020 at 4:42

3 Answers 3


Light, even in quantum mechanical form, travels as a wave. Photons as individual particles only exist at the end of propagation, when that wave interacts.

That wave always propagates at $c$, the speed of light in a vacuum.

So how does refraction work? As the wave moves through a medium, it intersects with (usually) the electrons, causing them to vibrate. That vibration does not exactly follow the wave: the E field causes a force, which causes an acceleration, which builds to velocity. That motion of charge causes the reradiation of another, weaker, delayed wave. The combination of the original wave and the reradiated one results in the overall wave being a bit delayed. The more material traversed, the more it’s delayed. In that sense, it’s showing a slower velocity: the more material it goes through, the longer it takes to get there.

Note that this is a continuous coherent shaping of a wave. It’s not the scattering of point-like particles like balls in a pachinko machine. This is consistent with what we see: there’s no random scattering of light going through clear optical glass

  • $\begingroup$ Thank you! I have a couple more questions, If I may. They're related to a fictional character(which was what drove me to post this thread). He can turn into light itself and/or shoot light based attacks. His attacks pack so much energy they casually pierced through solid objects or go as far as creating massive explosions. Does it change the rules? Could his light be simply obliterating atoms in the way instead of having them slow it down? Also, in his photons state, could he use already existing sunlight to travel without interruption at full speed, as if he were in a vaccum? $\endgroup$
    – BobiX
    Jan 14, 2020 at 6:41
  • $\begingroup$ It’s hard to know what a fictional character can and can’t do, particularly one who’s already beyond standard physics. Strong enough light can destroy materials, particularly if it can be short wavelengths (x-rays). But when that happens, the resulting plasma scatters the light so the character would end up pretty strongly disrupted. $\endgroup$ Jan 14, 2020 at 6:46
  • $\begingroup$ I was referring to a certain fictional character that can do said things. Of course, it defies out laws of physics, but he's not really subjected to all of them.The world that he exists in completely deviates from real life aside for some basic similarities. So I was wondering about things that might affect his ability, but the viewer cannot confirm(like the speed of his light in a medium). $\endgroup$
    – BobiX
    Jan 14, 2020 at 7:19
  • $\begingroup$ I upvoted the answer and edited my comment to not confuse the reader. The interaction with electrons cause them to vibrate, as mentioned in the comment. Note that electron has it’s ‘own’ charge, such that when it oscillates it produces changing E-field. As for your though experiment, if you would Imagine yourself as a photon, interaction from your point of view would never occur due to time dilation. Waves follow super position, so interaction of sunlight with electrons E-field would not keep ‘your’ light unaffected. $\endgroup$ Jan 14, 2020 at 7:37
  • $\begingroup$ @Bob Jacobsen So there is no explanation in terms of photon? $\endgroup$ Jan 14, 2020 at 11:56

Photons are massless and they do travel at speed c in vacuum when measured locally.

Though, in a medium, you can read phrases saying that the speed of light decreases. I do understand that you are confused.

If you want to go very basic, then you can say that the photons as they travel in the medium between the atoms and molecules, still travel in vacuum, at speed c. QM is a tricky beast because the photons do interact with the atoms/molecules in the medium. Now there are different approaches as to what kind of interaction this exactly is, that is, scattering (elastic or inelastic), or absorption and re-emission.

Without going into detail which one of those specific interactions is the real one (in reality we do not know), the interaction is there and needs time. The individual photons interact with the atoms/molecules in the medium, and this takes time. Though, the individual photons as they travel inbetween the atoms, they do travel in vacuum at speed c.

Nonetheless, the denser the medium is, the more interactions the photons have to have to propagate, and the more the speed of light slows down.

The simplest picture is that light always travels at the speed of light. But in a material it travels at the speed of light until it hits an atom. It is then absorbed and re-emitted in the same direction, which takes a small amount of time. The more this happens, the slower the effective average speed. The denser the material, the more atoms there are in the way.

Why does the light travel slower in denser medium?

It is the wavefront that slows down, and that is why we use classical phrases like the speed of light in a medium slows down.

Depending on the natural frequency of the atom and the frequency of the wave, the emitted photons will change phase when compared to the other, unaffected photons. Therefore, it either falls backward or forward a bit. Since this happens every time the wave hits an atom and there are many atoms in even the smallest piece of material, this has the affect of slowing the wavefront as the wave advances through the substance.

Change in Speed of Light

So each individual photon travels at speed c inbetween the atoms/molecules, thought the interaction with the atoms takes time, and this slows down the classical wavefront.

A photon is absorbed by one of the dielectric molecules, so, for a fantastically fleeting moment, it is gone. The absorbing molecule lingers for of the order of 10−15s in its excited state, then emits a new photon. The new photon travels a short distance before being absorbed and re-emitted again, and so the cycle repeats. Each cycle is lossless: the emitted photon has precisely the same energy, momentum and phase as the absorbed one. Unless the material is birefringent, angular momentum is perfectly conserved too. For birefringent mediums, the photon stream exerts a small torque on the medium. Free photons always travel at c, never at any other speed. It is the fact that the energy spends a short time each cycle absorbed, and thus effectively still, that makes the process have a net velocity less than c.

How does light speed up after coming out of a glass slab?

  • $\begingroup$ Thank you so much! So, photons do move at a constant speed, but their interactions while traveling just stall them, thus, they are technically slowed down in a sense that they take more time to cover said distance than if they didn't have to interact with anything. Hopefully I got that right. But I've got one more question, regarding a fictiona scenario: Let's say a man can turn into photons and travel, could he perhaps use the wave lenght of the sunlight to avoid interaction nad bypass the absorption effect? I'm probably talking out of my behinds, but figured I'd ask even if it's silly. $\endgroup$
    – BobiX
    Jan 14, 2020 at 2:41
  • $\begingroup$ @BobiX it is all probabilities, because it is QM, so even if you would be changing the wavelength (if this is what you are referring to), you might still get interaction with the atoms. $\endgroup$ Jan 14, 2020 at 2:57
  • $\begingroup$ Pardon my ignorance, I'm not that good with physics terms. I'm simply referring to if it's possible for him while in his photons state to use already existing, natural light produced by the sun to avoid interactions and travel as if he's in a vaccum. I know it probably depends on many variables, but since no further information is available, I can only assume there's air getting in his way, that's about it, unfortunately. $\endgroup$
    – BobiX
    Jan 14, 2020 at 3:11

The other answers explain how light appears to slow when it is slowed. But sometimes it isn't, and it was helpful for me to think of the limiting cases for light in water.

Imagine light entering water from a vacuum. Water has a molecular spacing on the order of a nanometer, so from the point of view of a photon with wavelength 1000 nm, the water is a sea of electrons. The photon interacts with the medium as a wave in a dielectric, and the other answers describe how wave-particle duality affects our understanding of the photon's travel.

Now imagine a photon with wavelength 0.01 nm -- an X-ray photon. To this photon, the water is mostly vacuum with some giant H2O particles wiggling around. Since the molecular spacing is much larger than the photon's wavelength, it will interact with the water more as a particle, traveling through the underlying vacuum until it scatters with a single H2O molecule. And we see this in the refractive index of water, which approaches 1 in the nanometer scale (blue line in plot below).

credit: philiplaven

  • $\begingroup$ Photons ALWAYS travel at the same speed. As photons enter different mediums they undulate between the atoms (similar to what your suggesting) but maintain the same speed. This simultaneously increases the path length and time they spend inside the medium. Higher energy photons would interact more with the atoms causing them to undulate (deviate) further from the path. Higher energy photons would therefor take a longer path through the material than a lower energy photon would. With this model you will NOT need a miraculous explanation for how photons speed back up after leaving the medium. $\endgroup$ Jan 14, 2020 at 18:19
  • $\begingroup$ @BillAlsept Correct, but I'll highlight "Higher energy photons would therefor take a longer path through the material than a lower energy photon would" -- it can be confusing for a student that realizes gamma rays take the shortest path. Your statement applies in the visible spectrum; outside of this energy regime it helps to reconsider the nature of light's wave-particle duality to explain why the basic rules from optics no longer hold. $\endgroup$
    – Sam
    Jan 14, 2020 at 18:44
  • $\begingroup$ higher energy photons deviate more as with shorter wavelengths bend the most. This is noticeable at the surface where angle of incidence affects the amount of refraction. Once inside the material the photon undulates back and forth equally so it maintains a relative straight trajectory other than bending back-and-forth around each atom. $\endgroup$ Jan 14, 2020 at 19:16

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