Why does long wavelength infrared not penetrate glass?

  1. Why does long wavelength infrared not penetrate glass?
  2. jcsd
  3. Glass is designed to only allow visible wavelength to pass, and once you get to the infrared spectrum glass simply absorbs it.

    (http://www.dandydesigns.org/id58.html this kinda explains it)
  4. If that is true, then we would not be able to feel the sun's heat standing beside a closed window.
  5. If I remmeber glass has impurities that block infrared, as pure SiO2 transmits it to some degree.

    Btw. the sun light can heat you through a glass window because other wave lengths can heat object too. Blue light is highly energetic foe exemple, an so if absorbed it gives a lot of heat, although in natural light it is not predominant by intensity (# of photons per second).
  6. SpectraCat

    SpectraCat 1,395
    Science Advisor

    Long wavelength infrared doesn't penetrate glass because of the molecular-level IR absorbance of the silica. Silica has a *very* strong band at around 1200 cm-1. This band blocks > 99% of the light at that frequency even at very low concentrations (i.e. a few mg of ground up glass in a KBr pellet). So, when you have the bulk material, that band is extremely broad, and produces complete extinction all the way out to ~2500 cm-1 (about 4 microns wavelength), and down to < 200 cm-1 (~ 50 microns). There is another strong band around 3500 cm-1 for the surface terminated Si-OH bonds, but there are no significant absorptions in the near-infrared (i.e. from 4000 cm-1 up to the visible, which starts around 13000 cm-1 or so). The latter point is why you can still feel radiant heat through a glass window pane, as you suspected.

    It should be noted that there are many different varieties of silica/glass/quartz, and all of them have different transmission properties, however, non of them transmit to any significant extent at photon energies below 2500 cm-1, as far as I am aware.
  7. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    It comes up a lot here - it's probably in the sticky thread, do a search for a more complete answer

    In simple terms a photon excites an atom (strictly a bond between atoms)
    This then de-excites emitting a new photon. the trick is that in a crystal the atoms are arranged in a regular pattern so the new photon goes off in the same direction.
    Glass is a bit tricky, the atoms aren't in a regular pattern like a diamond but they are fixed

    In a metal there are lots of free electrons on the surface which absorb any photons, but because these aren't in a regular pattern the emmitted photon goes off in a random direction - so metals are reflective.

    Also most commercial glass is also made so that any impurities which absorb visible light are removed, to make it as clear as possible.
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