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Thermodynamics

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Babinet Compensator

A quarter wave plate or a half wave plate produces only a fixed path difference between the ordinary and the extraordinary rays and can be used only for light of a particular wavelength. For different wavelengths, different quarter wave plates are to be used. To avoid this difficulty, Babinet designed a compensator by means of which a desired path difference can be introduced.

It consists of two wedge-shaped sections A and B of quartz. The optic axis is lengthwise in A and transverse in B. The outer faces of the compensator are parallel to the optic axis. Therefore, the ordinary and the extraordinary rays travel with different velocities along the same direction inside the compensator. Moreover, the extraordinary ray in A behaves as ordinary in B while the ordinary in A behaves as extraordinary in B. Suppose a plane polarized parallel beam of light is incident normally at the point C of the Babinet’s compensator. The beam is split up into extraordinary and ordinary rays. The path difference introduced between them after they have travelled a distance CD in A is . In B, the path difference introduced by B is .

Therefore, the resultant path difference

The crystals A and B are mounted such that A is fixed and B can slide along the surface of A with the help of a rack and pinion arrangement. In this way (t₁ – t₂) can be made to have any desired value. Hence any path difference can be introduced with the help of the Babinet’s Compensator and it can be used for light of any wavelength.