Retroreflection and Brilliance

It has been remarked that a faceted gemstone is a special purpose, retroreflecting prism. The object of faceting the pavilion at a particular angle is to allow as little light as possible to “leak” out the back of the stone, and to send this light, colored and dispersed, back to the viewer. In a retroreflector, light returns back to the direction of its source. A two-dimensional example is a billiards shot into a corner. The ball makes a bounce off one cushion, onto another, and leaves the corner, returning to the shooter neglecting spin, of course.

A three-dimensional example is easy to find. Take a close look at a taillight lens, or a highway or bicycle reflector. Notice that they are made of arrays of cube-corners. And notice at night how bright they are. Corner-cube arrays have been left on the surface of the Moon by …

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