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in Nature of Light by (57.3k points)

What do you understand by double refraction?

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Nicol Prism:

Nicol prism is device, made on the principle of double refraction, which is used to obtain the plane polarised light and it can also be used as analyser. In nicol prism, O-rays of two refracted rays is removed with the help of calcite crystal and only E-ray emerges from it as plane polarised light. Thus the plane polarised light is obtained by nicol prism.

Construction: To form a nicol prism a calcite crystal piece is taken such that its length be three times its width. The faces of width of this crystal are grinded in such a way that the angles of its section become 68° and 112° [Figure]. This crystal is cut in two pieces such that the cut plane become at right angle to main section. Both cut pieces are polished and then joined with the help of transparent cement of Canada Balsam. Upper and lower faces of obtained crystal are backened.

Working: The main section of a nicol prism, shown in figure, is ABCD. When an unpolarised ray of light PQ is incident on face AB, it breaks up in two refracted rays i, e., in O-ray and E-ray. Both the rays are plane polarised. Refractive index of calcite for O-ray is 1.658 and for E-ray it is 1.468. Refractive index of Canada Balsam is 1.55. Therefore for O-ray, Canada Balsam behaves as rarer medium while for E-ray it behaves as denser medium. When O-ray reaches Canada Balsam layer passing through the crystal, it goes from denser medium to rarer medium. Construction of nicol prism is done in such a way that the angle of incidence for O-ray on Canada Balsam becomes greater than 69°. Therefore, this ray is totally internally reflected and absorbed by the black pointed surface of the nicol prism. For E-ray, Canada balsam behaves as denser medium, therefore it emerges from the crystal which is plane polarised.

Nicol prism is used for obtaining plane polarised light and it can also be used as analyser.

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