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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.