Q. Describes the construction of a 'Nicol Prism' and explains its action in converting UPL into PPL.
Sol. Nicol Prism: This is the most simple and elegant polarizer and analyzer. It is an optical device made from a calcite crystal, based on principal of double refraction, and use to produce and analyze the PPL. In double refraction the incident light split into two components. One is ordinary, obey's all laws of refraction and move with the same velocity in all direction inside crystal known as ordinary ray.
The other one is extraordinary ray does not obey law of refraction and its velocity changes with direction inside the crystal.
In Nicol prism, O-ray is eliminated using optical means by 'Total internal reflection' and the transmitted ray is E-ray in which the vibration of electric field are in single direction i.e. light is PPL.
Construction: Nicol prism is constructed from a calcite crystal whose length is nearly three times of its width. The end faces of the crystal out down so as to reduce the angles in principal section to a more acute angle of 68° instant of 71°. The crystal is then cut along the diagonal and the two cut surfaces after grinding and polishing, cemented back together with a special cement called Canada balsam which is a transparent substance.
Working: Fig. shows the principal section of a Nicol prism. When light from source S is incident on surface PQ, at point M it divides into two refracted ray due to double refraction, one is ordinary and other is extraordinary.
The Nicol prism is constructed in such a way that O-ray strikes the Canada balsam layer at critical angle (69°) or more than this value. Since for the calcite µo = 1.468 and for Canada balsam µeb = 1.55, then the light travels from a denser to a rarer medium and incidence angle is equal or more than the critical angle hence the light gets totally reflected. Hence, the O-ray suffers TIR at Canada balsam layer. It then travels towards the blackened surface where it is absorbed.
However, extraordinary ray does not suffer TIR because for E-ray µe < µcb. As a result it comes out from the surface RT and PPL.
The detection of this PPL can be done by using another Nicol-prism.
Limitations:
1. Light must not travel along or perpendicular to the optic axis otherwise phenomenon of double refraction shall be missing.
2. It cannot be used for highly convergent or divergent rays.
3. The angle between the extreme rays of light incident on Nicol prism is limited to about 28°.