Wollaston Prism


Explanation of the structure and function of the Wollaston polarizing prism
The Wollaston prism consists of two orthogonal calcite prisms which are cemented together on their base. Their optical axes lie perpendicularly to each other and perpendicular to the direction of propagation of the incident light. Light striking the surface of incidence at right angles is refracted in the first prism into an ordinary (O) and an extraordinary (A) ray. However, these two rays continue to propagate in the same direction. As the optical axis of the second prism is perpendicular to that of the first, the ordinary ray (O) becomes an extraordinary ray (A) at the boundary surface. Its refractive index changes from n(O) to n(A); as n(A) < n(O) the extraordinary ray is refracted away from the axis of incidence. The opposite applies to the original extraordinary ray, now an ordinary ray: overall, therefore, the two partial rays display very different directions.


The Wollaston prism is a polarizing beam splitter, preserving both the O- and E-rays. It is usually made from calcite or quartz.


The Wollaston prism is made up of two right triangle prisms with perpendicular optic axes. At the interface, the E-ray in the first prism becomes an O-ray in the second and is bent toward the normal. The O-ray becomes an E-ray and is bent away from the normal. The beams diverge from the prism, giving two polarized rays. The angle of divergence of these two rays is determined by the wedge angle of the prisms. Commercial prisms are available with divergence angles from 15 to about 45. They are sometimes cemented with glycerine or castor oil, and sometimes not cemented if the power requirements are high.


Examples of Applications of Wollaston Prisms


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