Why does glass absorb infrared light? I was under the impression that glass was transparent to light with a wavelength above ultraviolet, but when playing with my latest toy, an infrared thermometer which brought up a question I asked here before, expecting to be able to measure temperature via a mirror, I found that it would essentially just give me room temperature (or mirror temperature), at most slightly more, while via a piece of shiny metal I was able to measure the temperature of the object of interest (at most slightly less). 
If it were the case that either most infrared radiation were transmitted or reflected I think I would be able to measure the temperature of the object, so I suspect that most heat radiation was absorbed. 
Why does that happen while it doesn't happen for visible light? I think that for high frequencies electronic energy levels are available to absorb light, and that their unavailability for light in the visible range made glass transparent.
Why is it that for infrared energy levels are available (mechanical?) that apparently are not available for higher energy visible light?
 A: As a general rule there are three mechanisms by which molecules absorb light:


*

*Electronic transitions - visible/uv wavelengths

*Vibrational transitions - infra-red wavelengths

*Rotational transitions - microwave wavelengths


In solids you don't often get rotational spectra because the molecules usually aren't free to move without interacting with the lattice, so you tend to get electronic transitions in the uv and vibrational transitions in the IR. It's probably not coincidence that there is frequently no absorption at visible wavelengths because we wouldn't have evolved eyes if there was.
In isolated molecules you get nice sharp vibrational transitions (with rotational structure as well) but in solids the interaction with the lattice tends to broaden out the absorption lines. You'll find numerous articles on IR spectroscopy of silica glass, for example this one though it's behind a paywall. As DumpsterDoofus comments, you get absorption due to hydroxyl and/or water, but you also get absorption due to various stretching modes of the Si-O-Si lattice.
