Question:
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.
Replies:
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).
NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.
