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From: MJackson.Wbst@XEROX.ARPA
Newsgroups: net.physics
Subject: Re: What polishing does to the surface of rocks.
Message-ID: <636@sri-arpa.UUCP>
Date: Thu, 2-Aug-84 09:50:00 EDT
Article-I.D.: sri-arpa.636
Posted: Thu Aug  2 09:50:00 1984
Date-Received: Sat, 4-Aug-84 04:03:38 EDT
Lines: 34

"Once the particle size becomes smaller than a value which is
approximately the wavelength of light. . .This is presumably the answer
to Gilman's question about
Xerox toner."

Wrong.

Xerographic toner varies somewhat in size but typical values lie in the
range of 5 to 15 microns.  Visible light has wavelengths in the range of
approximately 400 to 700 nanometers, or 0.4 - 0.7 microns.  Xerographic
toner is black because it absorbs the light incident upon it (mostly
because of the carbon black in the [heat-fusable] polymer resin).

The color of an object is the result of the way it interacts with
incident light.  One consideration is that to the extent that there is
front surface reflection the spectral energy density of the illuminant
is unchanged.  (Try looking at a glossy print--unless you hold it so
that direct reflections of light don't reach your eye the colors will be
washed out).  Light that isn't reflected at the front surface interacts
with the bulk of the object; this may involve absorption and
reradiation, absorption during transmission, reflection from internal
surfaces, etc.  This is how the SED of the light, and hence its
perceived color, is changed.

As to the original question, presumably rock dust scatters light at the
particle surface, thus not changing the color.  The random scattering
gives a dull, white (color of the illuminant) appearance.

By the way, when particles get small enough so that light no longer
interacts with them they appear TRANSPARENT, not black (no absorption).
That's why a solution is clear but a colloidal suspension is translucent
or opaque.

Mark