Date: Around 1999
Why are some things like glass clear, in that we can see
through them, while other obects like wood and not clear.
There are at least three effects to consider:
When light enters an object it can be absorbed so no light makes it through
Light reflects off the surface. This can be wavelength dependent like on
soap or oil films. Some colors are transmitted while others are reflected
from the surface.
Scattering of light:
When a substance is made of particles whose size is on the order of the size
of light wavelengths that substance will cause the light to scatter, resulting
in distortion of light that penetrates it. Thus light is transmitted but the
material is not transparent. For example, add a few drops of milk to water or
compare old, crazed, glass with new glass.
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Update: June 2012