The speed of light in a medium is the inverse of the refractive index of that medium. It's caused by the interaction between the electrons in matter (bound to a

The speed of light in a medium is the inverse of the refractive index of that medium. It's caused by the interaction between the electrons in matter (bound to atoms) and electromagnetic fields. It does not require inhomogenities larger than the atomic structure. https://en.wikipedia.org/wiki/Refractive_index https://en.wikipedia.org/wiki/Refractive_index