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Nicol Prism

It is an optical device used for producing and analyzing plane-polarized light. It was invented by William Nicol, in 1828, who was an expert in cutting and polishing gems and crystals. We have discussed that when a beam of light is transmitted through a calcite crystal, it breaks up into two rays: (1) the ordinary ray which has its vibrations perpendicular to the principal section of the crystal and (2) the extraordinary ray which has its vibrations parallel to the principal section.

The nicol prism is made in such a way that it eliminates one of the two rays by total internal reflection. It is generally found that the ordinary ray is eliminated and only the extraordinary ray is transmitted through the prism.

A calcite crystal whose length is three times its breadth is taken. Let A’BCDEFG’H represent such a crystal having A’ and G’ as its blunt corners and A’CG’E is one of the principal sections with ∠ A’CG’ = 70˚.

The faces A’BCD and EFG’H are grounded in such a way that the angle ACG becomes = 68˚ instead of 71˚. The crystal is then cut along the plane AKGL as shown in fig. the two cut surfaces are grounded and polished optically flat and then cemented together by Canada balsam whose refractive index line between the refractive indices for the ordinary and the extraordinary rays for calcite.

Refractive index for the ordinary, = 1.658

Refractive index for Canada balsam = 1.55

Refractive index for the extraordinary = 1.486

In fig. , the section ACGE of the crystal is shown. The diagonal AC represents the Canada balsam layer in the plane ALGK of fig.

It is clear that Canada balsam acts as a rarer medium for an ordinary ray and it acts as a denser medium for the extraordinary ray. Therefore, when the ordinary ray passes from a portion of the crystal into the layer of Canada balsam it passes from a portion of the rarer medium. When the angle of incidence is greater than the critical angle, the ray is totally internally reflected and is not transmitted. The extraordinary ray is not affected and is therefore transmitted through the prism. The working of the prism is clear from the following cases:

1. Refractive index for ordinary ray with respect to Canada balsam

∴ θ = 69˚

If the angle of incidence for the ordinary ray is more than the critical angle, it is totally internally reflected and only the extraordinary ray passes through the nicol prism. Therefore, a ray of unpolarized light on passing through the nicol prism in this position becomes plane-polarized.

2. If the angle of incidence is less than the critical angle for the ordinary ray, it is not reflected and is transmitted through the prism. In this position both the ordinary and the extraordinary rays are transmitted through the prism.

3. The extraordinary ray also has a limit beyond which it is totally internally reflected by the Canada balsam surface. The refractive index for the extraordinary ray = 1.486 when the extraordinary ray is travelling at right angles to the direction of the optic axis. If the extraordinary ray travels along the optic axis its refractive index is the same as that of the ordinary ray and it is equal to 1.658. Therefore, depending upon the direction of propagation of the extraordinary ray may be more than 1.55 and the angle of incidence will be more than the critical angle. Then the extraordinary ray will also be totally internally reflected at the Canada balsam layer. The sides of the nicol prism are coated with the black paint to absorb the ordinary rays that are reflected towards the sides by the Canada balsam layer.