BILLW@SRI-KL.ARPA (08/03/84)
From: William "Chops" Westfield <BILLW@SRI-KL.ARPA>
Most of this info comes from a solid state physics class I took
4 years ago, and Ive done little other than software since, but:
The optical behavior of a substance, whether whole, or finely
powdered deends very much on the substance. In particular, not
everything appears black if finely enough powdered (consider
cornstarch, sugar, powdered silica), nor does everything appear
white (consider platinum black, most other powdered metals).
Which happens in largely dependent on the electronic internal
structure of the material. Here are a few points:
o Conductors and dielectrics behave much differently.
For example, a dielectric will polarize a beam of
of light reflected from its surface, a conductor
will not (you can derive this from the continuity
of electric field requirments at surfaces, if I
remember correctly).
o Transparency is basically dependent on the band-gap
energy of the substance (ie how much energy it takes
to shove a typical electron into its next quantum
state). If the energy of the photon is less than this,
the photon does not interact with the substance, and
thus the substance is transparent. In conductors,
excited electrons can move around, again causing
quite different behavior than in dielectrics. In
semiconductors, things are still more complicated,
but this is where photovoltaic cells start from.
This also means that transparency is a function of
wavelength. For example, germanium is transparent
in the infrared region, though opaque to visible light.
All of this is overveiw and simplification, and doesn't explain
why powdered sand is white and powdered platinum is black. The
scattering due to many itty-bitty surfaces certainly has a lot to
do with it too. I'd be interested if anyone has a nice unified
theory! Something else to think about: When you polish metals,
they LOSE, rather than begin to show, their natural colors.
Polish them enough, and you have a very good mirror...
Enjoy BillW@SRI-KL.ARPA, ...menlo70!sri-unix!billw
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