Q: How do photochromic eyeglasses work? Do...
Q: How do photochromic eyeglasses work? Do they eventually wear out and just stay dark?
A: The glasses, which darken in sunlight and become transparent indoors, employ a chemical reaction to block sunlight. Glass is composed of transparent silicon dioxide. Photochromic lenses also contain a small amount of clear crystals of a light-sensitive chemical, such as silver chloride.
The silver chloride does not react to the normal light produced by lamps indoors. When exposed to the ultraviolet radiation of sunlight, however, it breaks apart into atoms of silver and chlorine. Free silver atoms absorb light and the glass darkens. When the wearer returns indoors, the atoms recombine and the glass becomes transparent. Silver chloride can go through this cycle indefinitely, so the glasses do not wear out.
