999. Why are any materials transparent? — MZ, Peligna, Italy
Because light is an electromagnetic wave, it is emitted and absorbed by electric charges. For an electric charge to emit light it must move—in fact, the charge must accelerate. For an electric charge to absorb light it must also move—it must also accelerate. However, there are many materials that do not have mobile electric charges. For example, while all electric insulators have electric charges in them, those electric charges can't move long distances. The electric charges in many electric insulators can't even move enough to absorb light and the light simply passes right through them. They are transparent.