Hi all.

Photons transitioning from air into yellow tinted water decelerate the same amount if entering green tinted water ?

Or, a red color light beam and a violet color light beam get different deceleration entering into clear water ? What changes crossing trough media ? Its energy, its wavelenght, both, other ?

Do photons crossing into a rainbow outputted by a prism experience several changes of speed according the color they are crossing ?

(high chance for poor terminology above 😟)

1 hour ago, Externet said:

Photons transitioning from air into yellow tinted water decelerate the same amount if entering green tinted water ?

Photons do not experience deceleration or acceleration. What happens in a medium other than vacuum is basically that they start interacting with electrons present there; you could perhaps say (not entirely rigorously) that they get absorbed and re-emitted, the process of which leads to a tiny but measurable delay. So the overall measured speed appears to be lower, even though the photons themselves always locally propagate at exactly c. But there is never any deceleration involved, since massless particles cannot travel at anything other than exactly c.

7 hours ago, Externet said:

What changes crossing trough media ? Its energy, its wavelenght, both, other ?

The wavelength.

9 hours ago, Markus Hanke said:

Photons do not experience deceleration or acceleration. What happens in a medium other than vacuum is basically that they start interacting with electrons present there; you could perhaps say (not entirely rigorously) that they get absorbed and re-emitted, the process of which leads to a tiny but measurable delay. So the overall measured speed appears to be lower, even though the photons themselves always locally propagate at exactly c. But there is never any deceleration involved, since massless particles cannot travel at anything other than exactly c.

I've never liked the 'absorption - delayed emission' explanation of refraction. Such an explanation would be expected to produce scattering as well as failing to account for the particular property of materials that produce higher refractive indices than other materials. I wrote the following about six months ago. It would appear that I thought I would need it again:

This is a common misunderstanding of refraction. Refraction is easiest to understand in terms of classical electromagnetic waves. This can then be translated to the quantum picture provided the important aspects of the classical picture are maintained. When an electromagnetic wave passes through a medium, it exerts a force on the charges and charge dipoles of the medium. Depending on how easily the charges and charge dipoles of the medium can move in response to this force, the motion of the charges and charge dipoles of the medium creates an electromagnetic wave that combines with the original electromagnetic wave to produce a total electromagnetic wave that is delayed with respect to the original electromagnetic wave and therefore travels through the medium at a slower speed. Thus, the refractive index of the medium depends on how readily the charges and charge dipoles of the medium can respond to the passing electromagnetic wave. This depends on the frequency of the passing electromagnetic wave. Higher frequencies exert a greater force, but larger bulkier charges and charge dipoles respond more to lower frequencies. At visible frequencies, only electrons can significantly respond to the passing electromagnetic wave, and in this case, the refractive index depends on the polarisablity of the electron orbitals of the medium and increases with frequency due to the increasing energy of the photons.

As for the question asked in the opening post about light passing through an absorbing medium, this can actually be somewhat complicated as one can get weird dispersion effects near absorption frequencies. These weird dispersion effects are sometimes reported as causing faster-than-c propagation of light through the medium. A proper understanding of the effects of dispersion requires an understanding of the notions of phase velocity and group velocity.

10 hours ago, Externet said:

Do photons crossing into a rainbow outputted by a prism experience several changes of speed according the color they are crossing ?

Light passing through light does nothing (photons at very high energy can scatter off of each other, but it’s rare)

14 hours ago, KJW said:

I've never liked the 'absorption - delayed emission' explanation of refraction. Such an explanation would be expected to produce scattering as well as failing to account for the particular property of materials that produce higher refractive indices than other materials.

I absolutely agree with you, which is why, in my post, I added the caveat that it wasn’t entirely rigorous. I chose to use it anyway as I thought it might be the best fit to what I perceived (perhaps incorrectly?) to be the level of background knowledge the OP possesses. It’s not always an easy task to balance technical rigour with the needs of the audience.

But +1 from me for the excellent explanation for what really happens 👍

21 hours ago, KJW said:

I've never liked the 'absorption - delayed emission' explanation of refraction. Such an explanation would be expected to produce scattering as well as failing to account for the particular property of materials that produce higher refractive indices than other materials. I wrote the following about six months ago. It would appear that I thought I would need it again:

This is a common misunderstanding of refraction. Refraction is easiest to understand in terms of classical electromagnetic waves. This can then be translated to the quantum picture provided the important aspects of the classical picture are maintained. When an electromagnetic wave passes through a medium, it exerts a force on the charges and charge dipoles of the medium. Depending on how easily the charges and charge dipoles of the medium can move in response to this force, the motion of the charges and charge dipoles of the medium creates an electromagnetic wave that combines with the original electromagnetic wave to produce a total electromagnetic wave that is delayed with respect to the original electromagnetic wave and therefore travels through the medium at a slower speed. Thus, the refractive index of the medium depends on how readily the charges and charge dipoles of the medium can respond to the passing electromagnetic wave. This depends on the frequency of the passing electromagnetic wave. Higher frequencies exert a greater force, but larger bulkier charges and charge dipoles respond more to lower frequencies. At visible frequencies, only electrons can significantly respond to the passing electromagnetic wave, and in this case, the refractive index depends on the polarisablity of the electron orbitals of the medium and increases with frequency due to the increasing energy of the photons.

As for the question asked in the opening post about light passing through an absorbing medium, this can actually be somewhat complicated as one can get weird dispersion effects near absorption frequencies. These weird dispersion effects are sometimes reported as causing faster-than-c propagation of light through the medium. A proper understanding of the effects of dispersion requires an understanding of the notions of phase velocity and group velocity.

Yes this expresses how I like to think of it. I have sometimes used the analogy of running on a trampoline, whereby you put energy into undulation of the surface, lending it energy and getting it back a bit later, but the net effect being that it makes it slower to run. This type of wave-based explanation accounts for the reduced phase velocity.

However, while it avoids claiming there is actual absorption and re-emission (and thus the scattering problem), what I struggle with is how to show this is consistent with the speed of photons under these conditions still being c. I am aware that (again in wave terms) the group velocity and the signal velocity will differ from the phase velocity in a polarisable medium, but I have yet to see any source claim that either of these corresponds to the velocity of the photons, or that either of these velocities remains equal to c within the medium.

How would you describe how it is that photons still travel at c in a polarisable medium, in spite of all this going on?

Edited December 27, 2025Dec 27 by exchemist

2 hours ago, exchemist said:

Yes this expresses how I like to think of it. I have sometimes used the analogy of running on a trampoline, whereby you put energy into undulation of the surface, lending it energy and getting it back a bit later, but the net effect being that it makes it slower to run. This type of wave-based explanation accounts for the reduced phase velocity.

However, while it avoids claiming there is actual absorption and re-emission (and thus the scattering problem), what I struggle with is how to show this is consistent with the speed of photons under these conditions still being c. I am aware that (again in wave terms) the group velocity and the signal velocity will differ from the phase velocity in a polarisable medium, but I have yet to see any source claim that either of these corresponds to the velocity of the photons, or that either of these velocities remains equal to c within the medium.

How would you describe how it is that photons still travel at c in a polarisable medium, in spite of all this going on?

If you explain it classically you don’t use photons. Not sure how you do it with photons without virtual-state absorption.

I like the classical wavefront explanation for the bending. Once light slows down the wavefront bends, since the part still moving fast shoots past until it, too, hits the slower medium. The new wavefront in the slower medium has turned toward the normal.

You can experience this with a car whose tires go off the road onto a more viscous surface. You get pulled to that side

18 minutes ago, swansont said:

If you explain it classically you don’t use photons. Not sure how you do it with photons without virtual-state absorption.

I like the classical wavefront explanation for the bending. Once light slows down the wavefront bends, since the part still moving fast shoots past until it, too, hits the slower medium. The new wavefront in the slower medium has turned toward the normal.

You can experience this with a car whose tires go off the road onto a more viscous surface. You get pulled to that side

Yes that's very familiar, but the interesting thing to a chemist is the deep connection between refractive index and the absorption and emission of EM radiation, through the polarisability of the medium and how close the light frequency is to an absorption band. For instance my understanding is that the reason why blue light is bent more than red in glass is because there is an absorption in the UV, so blue light experiences more refraction than red, as it is closer to the resonant frequency at which real absorption will occur. From this point of view it is a bit unsatisfactory to stick entirely to a classical picture, as absorption is quintessentially a quantum phenomenon.

I suppose one could envisage a fleeting pseudo-absorption followed by almost instant stimulated pseudo-emission. That would avoid the scattering issue that would arise if true absorption were to occur, as then spontaneous emission in random directions would be expected, as @KJW points out. So one would picture photons progressing at c, but in stop-start fashion, through the medium. In the trampoline analogy, you put your foot down and the surface gives, preventing forward motion, but then it rebounds, giving your input energy back and sending you on your way after a brief delay.

(I realise such analogies can't be pushed too far, but it is handy to be able explain the phenomenon is a way that gives non-experts some idea of the process.)

33 minutes ago, exchemist said:

Yes that's very familiar, but the interesting thing to a chemist is the deep connection between refractive index and the absorption and emission of EM radiation, through the polarisability of the medium and how close the light frequency is to an absorption band. For instance my understanding is that the reason why blue light is bent more than red in glass is because there is an absorption in the UV, so blue light experiences more refraction than red, as it is closer to the resonant frequency at which real absorption will occur. From this point of view it is a bit unsatisfactory to stick entirely to a classical picture, as absorption is quintessentially a quantum phenomenon.

But you can explain the classically as blue light has a higher frequency and interacts more strongly with the dipole (It’s a forced oscillator), and so the light slows down more. The dipole has a resonant frequency but you don’t have to talk about absorption.

A part of this is whether you talk about light vs photons.

In physics refraction is introduced well before quantum mechanics; it’s just based on classical concepts.