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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?

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    $\begingroup$ Much of the infrared absorption of standard glass is due in part to residual -$\text{OH}$ groups, which largely act like water in terms of their vibrational absorption spectra. In general, there is a big energy gap between vibronic transitions and purely vibrational transitions; as a result, most types of condensed matter in the universe tends to universally absorb large swaths of the deep ultraviolet spectrum (from vibronic excitation) and large chunks of the infrared spectrum (due to vibrational resonances). $\endgroup$
    – DumpsterDoofus
    Jun 25, 2014 at 0:23
  • $\begingroup$ For example, virtually any clear plastic will completely absorb deep UV (the energy goes into bond scission) and will cut out chunks of the IR spectrum, but there's a "goldilocks region" in the NIR-visible range between where the vibrational absorptions end and the vibronic absorptions begin, and some materials transmit there. So in essence, it's sort of a property that's inherent to how most chemical bonds work. $\endgroup$
    – DumpsterDoofus
    Jun 25, 2014 at 0:27
  • $\begingroup$ "Why is it that for infrared energy levels are available (mechanical?) that apparently are not available for higher energy visible light?" The fundamental vibrational frequency of most chemical bonds tops off around $3500\text{cm}^{-1}$, whereas visible light is around the $15000-25000\text{cm}^{-1}$ region; while it's possible to have overtone transitions that could end up that high, the higher harmonics are significantly weaker, and in real life are rarely (if ever?) strong enough to visibly color a material. $\endgroup$
    – DumpsterDoofus
    Jun 25, 2014 at 2:32
  • $\begingroup$ Thanks @DumpsterDoofus. Don't you want to make this into an answer? $\endgroup$
    – doetoe
    Jun 25, 2014 at 7:33
  • $\begingroup$ is it really absorbing? not just reflecting (like a mirror) ? $\endgroup$
    – flor1an
    Jul 22, 2017 at 17:19

1 Answer 1

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As a general rule there are three mechanisms by which molecules absorb light:

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.

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