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?