SMART MATERIALS—MATERIALS THAT ADAPT TO CHANGING CONDITIONS - Chemistry: Notable Research and Discoveries - page 111

PhotochromicMaterial—Changing
Colorinresponsetolight
Compounds that respond to light have long been known.
Some of these compounds are organic—which contain car-
bon and are associated with living organisms—and others
are inorganic. One of the most common uses of the inorgan-
ic compounds that respond to light is in photography. First
developed in the middle of the 19th century, photographic
fi lm or plates generally rely on compounds known as silver
halides—combinations of silver with one of the halogens (fl uo-
rine, chlorine, bromine, iodine, and astatine). Silver chloride,
a popular choice, turns dark when struck by light.
The response to light that occurs in photographic fi lms
is not reversible—once an exposed fi lm turns dark, it will
not change back. Irreversibility is not appropriate for eye-
wear (eyeglasses that could be used only once would hardly
be popular), but researchers eventually discovered a way
of embedding silver halides in glass or plastic that would
make them respond reversibly to electromagnetic radiation.
Ultraviolet radiation modifi es the atoms of silver to accept
electrons from the surrounding glass, resulting in a darken-
ing of the material. In the absence of radiation, the electrons
escape, returning the material to its transparent state.
The speed and extent of the reaction depend on the num-
ber of silver atoms and the composition of the lens. Other
photochromic compounds are sometimes used, though the
process is similar and is also reversible.
Photochromic lenses are oft en used in prescription eyeglasses—eye-
glasses that are worn to correct vision. Having photochromic lenses pro-
tects the eyes and, similar to sunglasses, shields the user from bright sun-
shine outdoors. Some prescription eyewear can be tinted like sunglasses,