A ball (radius $R$) has three layers. For $0<r<a$ it is a conductor with free charge $+Q$. For $a<r<b$ it is a linear dielectric $\epsilon$ with free charge embedded in it with density ...
We know that EM waves are slowed down in a dielectric. But at what speed does the photons that make up the wave travel? Do they always travel at the speed $c$, but colliding/being absorbed and ...