This article was originally posted at RealClearScience.

Those of us who wear glasses have long been familiar with Transitions lenses, a type of lens that darkens in sunlight but is clear while indoors. The technology relies on a chemical phenomenon known asphotochromism, in which molecules absorb a certain wavelength of light and change shape (that is,isomerize) in response. In the case of photochromic glasses, the lenses usually respond to UV light. That is why the lenses darken when exposed to the sun (which produces UV radiation), while they remain clear under artificial lights (which do not produce much UV light). Continue reading →