|MadSci Network: Physics|
Graham, Luster (or lustre) is the result of a combination of effects having to do with the way in which a material reflects light. Luster is somewhat of an imprecise term and many materials have what people perceive as luster. The first effect that causes luster is simply the reflection property of the surface material. That is, what happens to a ray of light when it hits the surface? A surface is called "specular" if it reflects light perfectly, like a mirror. In other words, the ray bounces off the surface at an angle that is opposite of the angle of incidence. A surface is called "lambertian" if it diffuses the light, like flat wall paint. In other words, the ray splits up at the surface and equal amounts of light are scattered in each direction. Most surfaces are part specular and part lambertian, giving them their unique reflective properties by the proportion of each. Generally, the more specular a surface is, the more strongly we perceive a luster. Highly polished metals are specular, which gives them their characteristic luster. Another effect that impacts luster is the penetration of light into the material. Light will penetrate into some materials more than others. In addition, some light will penetrate while other light may bounce, depending upon its angle of incidence to the surface. Penetration depends upon a number of factors, such as the transparency of the material to various wavelengths of light and the orientation of the molecules within the material. As with reflectance, transparency tends to be a continuum from very transparent, like glass, to opaque, like brick. As a light ray penetrates a material, it eventually may bounce off of a molecule of a different material or encounter a change in molecular orientation. The light may be absorbed or reflected back, or even absorbed and reemitted at a different wavelength. All of these factors affect perceived luster. Compare the luster of a pearl to that of an opal. Both are combinations of reflection and penetration and other effects. Compare the luster of a fancy automobile with many layers of clear gloss lacquer to that of one with only a factory paint job. The layers of gloss add to perceived luster as a "depth" in the paint, because some light (at low incidence angles) reflects off the surface, while other light penetrates and reflects "deeper" into the finish. Cheers, Todd
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