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First, I'm not sure if photons have to "get up to" the speed of light, or if they are thrown into existence at that speed. I know that they should just be generated moving at their speed, and I know that they have zero mass so asking about acceleration is a little strange, but what about when light hits an interface? Does it need to "slow down" to the new speed of light? Or does a new photon get generated?

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@Qmechanic - That post doesn't present a concrete answer. That's just a bunch of answers where everybody disagrees with each other. I think it makes sense to have this specific question. :) –  New_new_newbie Jun 23 at 5:44

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Does it need to "slow down" to the new speed of light? Or does a new photon get generated?

One has to keep clear in this case the difference between a photon and an electromagnetic wave.

An individual photon is an elementary particle, its wavefunction is given by the quantum mechanical form of Maxwell's equations and the square of this wave function is a probability distribution for its position in space time, not a wave in space.

The electromagnetic wave, is a solution of the classical Maxwell equations and is the one that defines the different indices of refraction going through media and the different velocity c' observed while going through a medium.

The electromagnetic wave is built up by a great ensemble of photons where the frequency of the classical wave is the same as the frequency of the probability wave, the one that describes the energy of the photon, E=h*nu.

What the individual photons do when in ensemble is that they join up so that the phases match up and one observes the classical wave. Crossing a medium individual photons may,for example, scatter elastically from the field of the crystal and change the phase relations in the ensemble, building up the refracted wave.

If one were able to tag an individual photon one would find its velocity c, but the group and phase velocities building up the classical wave differ before and after crossing the surface of the medium. Each individual photon changes phase in the (transparent) new medium. The changes are such that the ensemble of photons builds up the classical wave which fulfills the refraction index behavior.

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Light in a vacuum (and according to special relativity and electromagnetism) does not accelerate, once it is generated it already has a speed of $c$ (constant speed of light).

However in anisotropic media the light speed can be different. In general relativity the effect of gravitational lensing is observed (and predicted).

Whether this is an acceleration (eg as in mechanics) see this answer, however this can be an open question in such cases (as general relativity itself can be an open question)

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What's with the down votes without comments? It's not helpful to anyone! –  Anthony Jun 23 at 6:13
    
@Anthony, sorry i dont understand, what do you mean? –  Nikos M. Jun 23 at 6:21
    
@NikosM. Someone downvoted this answer, probably thinking it is bad, but didn't explain why. My guess: it's not clear what "anisotropic media" (glass/water?) have to do with general relativity. You probably have to expand the section on anisotropic media and separate it from GR. –  user27542 Jun 23 at 18:54
    
Going from vacuum into a medium with a smaller light speed is conceptually ok, you can have the photons/EM waves being absorbed and re-emitted. But coming out the other way, as you get near the interface, the probability of absorption will become very small, yet the photons speed up! Great question. –  Rob Jeffries Jun 24 at 17:58
    
@Anthony, ok, the answer states different cases (anisotropic media, GR) where the light speed can be different. Whether GR metric can be seen as some variation of anisotropic media (at least for some cases) is another issue (i would say yes) –  Nikos M. Jun 24 at 19:03

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