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Refractive index μ = tan i p = 1 / sin C where , C = critical angle .
Double Refraction
When unpolarised light is incident on a calcite or quartz crystal it splits up into two refracted rays . one of which follows laws of refraction . called ordinary ray ( O-ray ) and other do not follow laws of refraction . called extraordinary ray ( E-ray ). This phenomena is called double refraction .
Dichroism
Few double refracting crystals have a property of absorbing one of the two refracted rays and allowing the other to emerge out . This property of crystal is called dichroism .
Polaroid
It is a polarising film mounted between two glass plates . It is used to produce polarised light . A polaroid is used to avoid glare of light in spectacles .
Uses of Polaroid
( i ) Polaroids are used in sun glasses . They protect the eyes from glare .
( ii ) The polaroids are used in window panes of a train and especially of an aeroplane . They help to control the light entering through the window .
( iii ) The pictures taken by a stereoscopic camera . When seen with the help of polarized spectacles , create three dimensional effect .
( iv ) The windshield of an automobile is made of polaroid . Such a mind shield protects the eyes of the driver of the automobile from the dazzling light of the approaching vehicles .