Birefringence and optical rotation

A substance is anisotropic, if it has different properties in different directions. If the refraction index n differs for perpendicular and parallel-polarised light (n and Hy) the result is birefringence or double refraction (when transparent). Accordingly, birefringence is evidenced by the ability of a material to rotate the plane of polarised light. It is defined as the difference in refractive indices in the directions parallel and perpendicular to the direction of orientation  

Birefringence may be a natural property or artificially induced by means of force fields. 

Natural birefringence is mainly found in crystalline substances artificial birefringence is normally based on mechanical orientation, either in the solid or in the liquid state stretch and flow orientation, respectively. Melts and solutions may show birefringence, caused by  

Reliable data on these phenomena are scarce, so that our discussion will be mainly qualitative. 

Related to birefringence is the optical activity or optical rotation (of the plane of polarisation of the light). This also may be a natural or an artificially aroused effect. 

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