# Is the speed of photon always $c_0$? [duplicate]

The propagation of light in medium is sometimes interpreted as the photons moving with $c_0$ (the speed of light in vacuum), occasionally absorbed by particles and released again by transition, causing a time lag.

Is this the generally accepted explanation of why the light appears to travel with a speed $c<c_0$ in medium, or is it just a way to make students understand the interaction between photon and medium? Will the speed of photon be affected by external fields ($E$, $M$, $G$) in a medium?

This is a simplified description that is appealing and not too wrong for high school students. It was greatly popularised by Richard Feynman in his lay book "QED: The Strange Theory of Light and Matter" as a means of introducing the idea of a medium. The more accurate description is that the electromagnetic disturbance in a medium is a quantum superposition of excited matter states and free photons, the latter still with speed $c$. If you think of the phenomenon as sequential, i.e. with the disturbance as a sequence of propagation - absorption - re-emission cycles, then it is mysterious as to how the re-emission is in the same direction as the incident light. You need to understand that the whole lot: free photon states and excited matter states are in superposition. Therefore, the emitter can in principle emit in any direction, but it has a low amplitude to do so owing to destructive interference with the other coupled states in the superposition unless the re-emission matches the incoming photon direction closely.
In summary, although my view is often thought of pedantic by my work collegues, the locally measured speed of a photon is indeed always the universal massless particle velocity $c$. Light in a medium is not pure "light", it is a quantum superposition that, if you want to be more precise, is variously called "polariton", "plasmon" or "exciton" depending on the exact nature of the exited medium states in the superposition.