Glass transparence

Basically there is nothing to absorb the light.
The electron transitions in glass do not match the energy of visible light and so it goes straight through - the same mechanism that makes water transparent.

Other substances in the glass can change the absorption, water strongly absorbs infrared so to make glass fibre-optic for infrared you have to remove all the water from the molten glass first.

 

Photochromatic is a dye molecule that gets broken by UV light, this leaves a bunch of bonds with lots of loose electrons that can then change energy levels and absorb visible light.
The clever bit is finding a dye that heals itself when you take it out of UV.

 

I have a block of formerly transparent lucite that was irradiated with a 15 MeV electron beam in 1959. It was a dark brown due to F-center dislocation. Now, 50 years later, it is transparent again, due to thermal (room temperature) annealing.

For glass to be truly transparent (no absorption), there can be no dispersion (See Kramers Kronig or dispersion relations). See
http://en.wikipedia.org/wiki/Kramers-Kronig_relations
These also apply to the real and imajinary components of electrical circuits.