So today, an interesting question was raised in my Optics lecture.We know that velocity of light in medium is less than . Therefore, we can move in an inertial frame where the photon has no velocity. Hence it would be possible to compute the rest mass of a photon. (Which is obviously not true, as photon has no mass.) Where are we going wrong? Further, adding to the confusion, we have cherenkov radiation which says that a charged particle traveling at a speed greater than the "phase velocity" photon emits electromagnetic radiation which gives rise to the blue color in the above picture. Answer: Thanks to a senior of mine. He was able to give me an outline of what is the reason. It is often misunderstood that velocity of light changes in the medium. But in effect, this isn't true. The velocity of light is and always will be in any medium. Just that, when the photon enters the medium, the dielectric of the medium absorbs the light and emits another photon. If the medium is a crystal, due to its isotropic nature, the absorption and emission of light is more systematic and the effective velocity due to the time delays between this process gives a "net" reduction of velocity of light. This is an outline of what happens but a clear understanding can be got by looking more into the quantum mechanical aspects of refraction. Kudos!