Originally posted by garytse86 Does anyone know why glass is transparent?

It is rather simple to explain why crystalline SiO2 is transparent... it's because of the atomic spacing. The periodic potential well generated by the crystalline structure gives rise to electronic band-gaps. Essentially band-gaps represent energies in which the electrons in the valence electrons will not be excited to a higher state. In crystalline metals and most semiconductors, light in the visible range will excite these electrons so this energy will be dissipated before it passes through the material. In some common crystalline insulators, the atoms have a larger atomic spacing which gives rise to a larger band gap. The band gap of such materials requires a larger energy than provided by visible light in order for electron excitation to take place. Therefore, the light passes through the material without interaction.

Recall that glass is simply amorphous SiO2. In a mathematical sense, it is somewhat more difficult to explain why glass is transparent, but the polymer chains essentially exhibit a similar order to the crystalline SiO2... but for a shorter range. On a large scale, amorphous SiO2 has similar opticl properties to its crystalline form.

From quantum mechanics, we learned that an electron can only absorb light if that absorption allows it to move to another energy level. In solid state, we learn that crystal lattices give rise to whole bands of allowable energies for transitions. These energy bands dictate the absorption spectra of the material.

But, as noted, something is happening to the light that is not absorbed. It is slowed. That is our working definition for index of refraction - how fast does light travel through the material. Since we are also taught that light has only one speed, c, something funny must be going on.

It is. Light of an energy that can not nominally be absorbed does get absorbed. This absorption gives the local matter no allowable state to change to, so it re-emits the light. The delay between absorption and re-emission causes the apparent speed to change. The likelihood of these aborted absorptions per unit length travelled is one factor that affects index of refraction.

The band gap answer is correct of course, but this is not all. The other point is that glass is an amorphous solid.

In crystals there are almost always imperfections that will scatter light: grain boundaries, inclusions, cracks, etcetera. It is much easier to make a large volume of glass that is uniform on the scale of the wavelength of visible light.