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Question
Discuss about Nicol prism.
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Solution
Principle: Double Refraction
Construction:
- One of the most common forms of the Nicol prism is made by taking a calcite crystal which is a double refracting crystal with its length three times its breadth.
- It is cut into two halves along the diagonal so that their face angles are 72° and 108°.
Nicol Prism - The two halves are joined together by a layer of Canada balsam, a transparent cement.
- A ray of unpolarized light from a monochromatic source such as a sodium vapour lamp is incident on the face AC of the Nicol prism. Double refraction takes place and the ray is split into ordinary and extraordinary rays.
- They travel with different velocities.
- The refractive index of the crystal for the ordinary ray (monochromatic sodium light) is 1.658 and for the extraordinary ray is 1.486. The refractive index of Canada balsam is 1.523.
- Canada balsam does not polarise light.
- The ordinary ray is total internally reflected at the layer of Canada balsam and is prevented from emerging from the other face.
- The extraordinary ray alone is transmitted through the crystal which is plane polarised.
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Uses of Nicol prism:
- It produces plane polarised light and functions as a polariser
- It can also be used to analyse the plane polarised light (i.e) used as an analyser.
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Drawbacks of Nicol prism:
- Its cost is very high due to the scarcity of large and flawless calcite crystals
- Due to an extraordinary ray passing obliquely through it, the emergent ray is always displaced a little to one side.
- The effective field of view is quite limited
- Light emerging out of it is not uniformly plane polarised.
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