UV activates darkening sunglasses
Q:  How do Photogrey sunglasses work?
A:
When you don the Photogrey sunglasses inside, they're as clear as glass. But, as you step outside-maybe into a hot desert
landscape-ultraviolet (UV) rays careen around, hit your skin, reflect into your eyes, and impact the glasses lens. In response,
the glasses darken and block up to 80 percent of the light radiation within a minute. The UV block helps protect your eyes
from cataracts and other damage.
Right: [Corel Corporation]
The sunglasses can change colors under light (photochromic) because of a special manufacturing process. When the glass is
molten, workers add tiny, transparent crystals of silver halide.
The silver-halide crystals change chemically when substantial amounts of ultraviolet light hit the glasses. The silver ions in
the crystals combine with an electron to form silver atoms inside the glass. The jagged, imperfect silver particles cluster
together, like the silver specks that form developing black and white film. The silver blocks light and the glasses darken.
The stronger the ultraviolet, the darker the gray.
Halogen gas, released when the silver ions formed atoms, cannot escape the glass. When you remove the glasses from the UV light by stepping inside, the
trapped halogen gas atoms recombine with the silver atoms and form clear silver-halide crystals once more.
The dark glass clears and becomes transparent
again.
(Answered by April Holladay, science correspondent, July 4, 2001)
Further Surfing:
U of Hawaii: Photochromic glass
HowStuffWorks.com: Photochromic sunglasses
HowThingsWork.com: Photochromic sunglasses
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