The word photo refers to light and chrome refers to color. The change in color with exposure to light is the basic principle of photochromatic glasses. Eyeglasses or sunglasses that darken when exposed to the sun are known as photochromatic glasses. Photochromic lenses are activated by UV radiations. When exposed to sun, on an account of specific chemical reaction they darken.
Early photochromic lenses were made of millions of molecules of substances like silver chloride or silver halide. Modern photochromic lenses tend to be plastic and instead of silver chemicals they contain organic (carbon-based) molecules that react to light. When exposed to ultraviolet rays in sunlight, the molecules undergo a chemical process that causes them to change shape. They are rearranged in such a way that they start to absorb the visible light. As there are millions of chemical reactions taking place, the significant absorption of visible light takes place and so the lens darkens. When the intensity of the ultraviolet rays is high, darker becomes the lens.
When the glass is worn inside a car where the UV light is absorbed by the windshield of the car, the reverse chemical reaction occurs. The molecules are transparent to visible light and they change back to their original shape, resulting in the loss of their light absorbing properties and returning to their clear state.
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