Bit of a silly question I know, but I have never been able to get answer which I'm happy with. I thought it might be due to the energy levels available to the electrons in the glass. Light is able to pass through the material, because the photons in the visible part of the spectrum don't have a high enough energy to excite the electrons in the glass. However if this is the case it should be perfectly transparent. Yet a piece of glass both refracts and reflects light. So light must interacting with the glass by some mechanism. Does anyone have the answer?