The lenses of photochromic spectacles are made of glass mixed with minute crystals of light sensitive silver iodide or bromide. These crystals are so minute that they have length of only 50 angstrom (1 angstrom = 0.0000000001 meter). As these crystals split and scatter in sunlight the transparent lenses automatically start becoming somewhat dark. Eyes are not dazzled because some of the sunrays are obstructed by these crystals. The crystals remain embedded in the lenses. Unlike a liquid when strained flows out of the strainer the photochromic glass does not allow these scattered crystals to flow out. As soon as direct exposure to sunlight is withdrawn the crystals of silver iodide or bromide fuse together and the lenses become transparent again.
Photochromic glasses were invented by a chemist named Dr. R.H. Dalon and were first manufactured in 1964 by American company named Corning.
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