Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!mgnetp!ihnp4!zehntel!hplabs!sri-unix!MJackson.Wbst@XEROX.ARPA 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