So normally we explain transparency by saying that the band gap in insulators is too large for mere visible light photons to be absorbed by the electrons. But what about the classical EM explanation? I made an attempt at an explanation below. Feel free to correct mistakes. Since Insulators lack free electrons, only elastic scattering by the electron clouds surrounding each atom will occur and hence light is simply refracted. In a conductor, on the other hand, there are lots of free electrons that absorb and re-emit the light in such a manner that it gets converted into heat. Hmm, but then why do metals reflect light so well instead of just absorbing everything? Can this stuff only be explained by mathematics and use of maxwell's equations?