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The basic principle behind Nicol Prism is based on its unique behaviour on the event of incidence of light rays on its surface.
When an ordinary ray of light is passed through a calcite crystal, it is broken up into two rays:
- An ‘Ordinary ray’ which is polarized and has its vibrations perpendicular to the principle section of the crystal and
- An extra-ordinary ray which is polarized and whose vibration is parallel to the principle section of the prism.If by some optical means, one of the two rays eliminates, the ray emerging through the crystal will be Plane polarized. In Nicol Prism, ordinary ray is eliminated and Extra-ordinary ray, which is plane polarized, is transmitted through the prism.
A calcite crystal’s length is three times its breadth. Let ADFGBC be such a crystal having ABCD as a principle section of the crystal with BAD = 700.
The end faces of the crystal are cut in such a way that they make angles of 680 and 1120 in the principle section instead of 710 and 1090. The crystal is then cut into two pieces from one blunt corner to the other along two pieces from one blunt corner to the other along a plane perpendicular to the extra ordinary rays.
- Refractive index of Calcite for O ray,
- Refractive index of Canada balsam,
- Refractive index Calcite of E ray,
Thus we see that the Canada Balsam is optically denser than calcite for E ray and rarer for O ray. Finally the crystal is enclosed in a tube blackened inside.
Unpolarised light incidence
When a ray SM of unpolarised light parallel to the face AD’ is incident on the face AB’ of the prism, it splits up into two refracted rays, the ordinary ray and the extra ordinary. Both of the O and E ray are plane polarized the vibrations of O ray being perpendicular to the principal section of the crystal; while that of E ray being in the principal section. The ordinary ray in going from calcite to Canada Balsam travels from optically denser medium to a rarer medium.
As the length of calcite crystal is large, the angle of incidence at Calcite – Balsam surface for the ordinary ray is greater than the critical angle. Therefore when O ray is incident on Calcite – Balsam surfaces it is totally reflected and is finally absorbed by the side AD’ which is blackened. The extra ordinary ray travels from an optically rare medium to a denser medium, therefore it is not affected by the Calcite – Balsam surface and it is therefore transmitted through the prism. This “E ray is plane polarized and had vibration, in the principal section parallel to the shorter diagonal of the end face of the crystal. Thus by Nicol prism we are able to get a single beam of place polarized light. Thus Nicol prism can be used as a polarizer.
When the angle of incidence at the crystal surface is increased, the angle of incidence at Calcite – Balsam surface decreases. When the angle S0MS becomes greater than 14o, the angle of incidence of Calcite – Balsam surface becomes less than the critical angle. In this position ordinary ray is also transmitted through the prism along with extraordinary ray so light emerging from Nicol prism will not be plane polarized.
When angle of incidence at crystal surface is decreased, the extraordinary ray makes less angle with the optic axis, as a result its refractive index increase, because the refractive index of calcite crystal for E ray is different in different directions through the crystal being maximum when the E ray travels at right angles to the optic axis and minimum when E ray travels along with O ray and no light emerges from the prism
Nicol prism as analyzer
Consider two Nicol prisms arranged coaxially one after another. When a beam of unpolarized light is incident on the first prism P, the emergent beam is plane polarized with its vibrations in principal section of first prism. This prism is called polarizer. When principal section of both prisms are parallel then intensity of emergent light is maximum. But when the principal sections are at right angles to each other the, intensity of emergent light is minimum i.e., there no light it transmitted through the second prism. Here first prism produced plane polarized light and 2nd prism detects and analyses it.