My mental model is that of the EM field coupling with the internal EM fields of a material to give rise to the phenomenon of index of refraction where light appears to move slower than the speed of light in a vacuum in said material.
As I understand, a more advanced version of this occurs in superconductors which serves as a much better model of the phenomenon. At least I'm told it would if I could claim to understand it!
https://physics.stackexchange.com/questions/33240/how-come-a...
https://physics.stackexchange.com/questions/47791/what-do-ma...
