Question

Discuss about Nicol prism.

Answer 1

Principle: Double Refraction

Construction:

1. 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.
2. It is cut into two halves along the diagonal so that their face angles are 72° and 108°.
   
   Nicol Prism
3. The two halves are joined together by a layer of Canada balsam, a transparent cement.
4. 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.
5. They travel with different velocities.
6. 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.
7. Canada balsam does not polarise light.
8. The ordinary ray is total internally reflected at the layer of Canada balsam and is prevented from emerging from the other face.
9. The extraordinary ray alone is transmitted through the crystal which is plane polarised.

• Uses of Nicol prism:

1. It produces plane polarised light and functions as a polariser
2. It can also be used to analyse the plane polarised light (i.e) used as an analyser.

• Drawbacks of Nicol prism:

1. Its cost is very high due to the scarcity of large and flawless calcite crystals
2. Due to an extraordinary ray passing obliquely through it, the emergent ray is always displaced a little to one side.
3. The effective field of view is quite limited
4. Light emerging out of it is not uniformly plane polarised.