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Name:  Bruce
Status:  educator
Age:  30s
Location: N/A
Country: N/A
Date: 2000-2001

Why do window panes block infrared and UV rays but permit visible light? Visible light wavelengths are in between these two part of the EM spectrum.

The wavelengths of electromagnetic radiation -- whether U.V., visible, or infra red and beyond is full of windows that will allow the transmission of the radiation, and absorptions that prevent the transmission of the radiation. The wavelengths at which these holes and absorptions occur depend on the detailed chemistry and physics of the particular substance. Glass happens to absorb U.V. radiation shorter than approx. 300 nm and longer than about 700 nm, but is transparent to wavelengths in between.

Vince Calder

Electromagnetic radiation can interact with matter in a couple of ways. One way is to interact with the rotational energy of the molecules as they rotate in their environment. This is a fairly low energy interaction and occurs fairly far into the IR. Another way is to interact with the vibrational energy of the atoms within the molecules. This requires energy in the near IR. A third way is to interact with the electronic energy of the chemical bonds or of individual electrons in an elemental substance. This requires a large amount of energy and typically occurs in the UV. For substances that are colored this occurs at the somewhat lower energies of visible light.

Glass can have vibrational interactions with light, causing it to block IR. It can also have electronic interactions with light, causing it to block UV. It does not have the low energy electronic interactions with light that lead to color materials (which results from blocking some wavelengths of visible light). These visible light interactions typically require delocalized electrons (in the case of organic molecules) or involve d-orbital transitions (which are closely spaced in energy).

Greg Bradburn

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