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QUESTION : How does a photo chromatic glass turn dark when exposed to sunlight?

V.Siva Prasad, Kerala

ANSWER: Photo chromatic glasses undergo change of colour on exposure to ultra-violet rays.

The shortest wave length of ultra- violet (UV) radiation from the sun which reaches the earth's surface is 208 nm.

Between this and the visible wave lengths (380 nm to 770 nm- violet to red) the ultra violet is divided into UVA and UVB.

UVA which enters the eye is mostly absorbed by the human lens, which causes cataract.

Photo chromatic glass undergoes a change by incorporating sub microscopic crystals of substances such as silver halides which turn dark in colour, while absorbing white light.

In general any photo chromatic material is a good UV absorber. Their main function is to interfere with the visible rays.

The time taken for the glass to change colours ie from dark to light differs with every glass.

Photo chromatic glasses can be used for correcting any type of vision and can be worn like ordinary glasses.

V.Ganapathy,Tiruchi, T.N.

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