Nicol prism

The phenomenon of double refraction of certain crystals can be used to produce plane polarized light. As we know the double refracting crystal split two different ray having two different state of polarization . say E-ray and O-ray . It is consist of calcite crystal with a layer of Canada balsam. for E-ray refractive index 1.486 and for O-ray refractive index 1.658 the refractive index for Canada balsam 1.55 which is lie between refractive index of calcite crystal of E-ray 1.48 and for O-ray 1.65 . 

The observation of the values of refractive indices reveal that the Canada balsam is a denser medium than calcite for E-ray and is a rarer medium than calcite for O-ray 

The calcite crystal is taken in the form of parallelogram with grounded edges with angle 109 degree and 71 degree . Due to grounding the acute angles get reduced to 68 degree. 
The light is incidents such that O-ray [going from denser to rarer medium] suffers total internal reflection and get absorbed in the backended sides. But E-ray is going from rarer to denser medium therefore it get transmitted .i.e. the transmitted ray is plane polarized

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