Photochromism

Photochromism does not have a rigorous definition, but is usually used to describe compounds that undergo a reversible photochemical reaction where an absorption band in the visible part of the electromagnetic spectrum changes dramatically in strength or wavelength.

http://en.wikipedia.org/wiki/Photochromism

Researchers in Japan are reporting development of a new so-called "photochromic" material that changes color thousands of times faster than conventional materials when exposed to light. The development could lead to a wide range of new products including improved sunglasses, more powerful computers, dynamic holograms, and better medicines, the researchers say. Their report appears in the Journal of the American Chemical Society, a weekly publication.





Presto, Instant Sunglasses!

Researchers have developed a material that almost instantaneously changes from clear to dark blue when exposed to ultraviolet (UV) light, and it just as quickly reverts to clear when the light is turned off. The new material, one of a class called photochromics, could be useful in optical data storage as well as in super-fancy sunglasses.

http://news.sciencemag.org/sciencenow/2009/04/24-01.html