Originally Posted by raymor

Something like the winding road. The photons travel at the same speed. We know that some substances, like wood, absorb light. At the particle level, that means photons are absorbed by the molecules in the wood.

In other substances, we day that light is transmitted through. In glass, the individual photons are briefly absorbed, then re-emitted. The rate of absorption and re-emission controls how long it takes for a photon to make it all the way through the material.

So while the photons move at velocity C, the time spent on absorbtion reduces the average velocity.