PPT Slide
don’t travel at speed c ?
It depends on how you want to think about it.
You can say that the photons occasionally interact with atoms in a dielectric, being absorbed and re-emitted, and that this only appears to slow them down. Here, the photons travel at “c”
Or you can say that the wave that propagates through the solid is a combination of a photon and virtual excitations of the atoms of the solid. This wave travels with v<c.