Double Image Polarizing Prisms

Nicol prism cannot be used with ultraviolet light on account of the Canada balsam layer which absorbs these rays. Sometimes it is also desirable to have both the ordinary and the extraordinary rays widely separated. For this purpose two prisms viz. (i) Rochon prism and (ii) Wollaston prism are used.

(i) Rochon prism: It consists of two prisms ABC and BCD (of quartz or calcite) cut with their optic axes as shown in fig. The prism ABC is cut such that the optic axis is parallel to the face AB and the incident light. The prism BCD has the optic axis perpendicular to the plane of incidence. Light incident normally on the face AC of the prism passes undeviated up to the boundary BC. In the prism BCD, the ordinary ray passes undeviated. If the prisms are made of quartz, the extraordinary ray is deviated as shown in fig. In the case of calcite, the extraordinary will be deviated to the other side. The prisms ABC and BCD are cemented together by glycerine or castor oil. Here, the ordinary emergent beam in achromatic whereas the extraordinary beam is chromatic.

(ii) Wollaston prism: It consists of two prisms ABC and BCD of quartz or calcite cut with their optic axis as shown in fig. They are cemented together by glycerine or castor oil.

A ray of light is incident normally on the face AC of the prism ABC. The ordinary and the extraordinary rays travel along the same direction but with different speeds. After passing BC the ordinary ray behaves as the ordinary while passing through the prism BCD. One ray is bent towards the normal while the other is bent away from the normal. In quartz > 0. Therefore, the ordinary ray while passing the boundary BC is refracted towards the normal as an extraordinary ray while the extraordinary ray is refracted away from the normal ray as an ordinary ray as shown in fig. If the prisms are made from calcite, the directions of the ordinary ray and the extraordinary rays are interchanged. While coming out of the face BD of the prism, the ordinary and the extraordinary rays are diverged. The prism is useful in determining the percentage of polarization in a partially polarized beam. Double image prisms are used in spectrophotometers and pyrometers.

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