In an isotropic medium, the velocity of propagation of light is independent of the direction of propagation. However, in anisotropic media, the velocity depends upon the direction in which light is propagating. Also, it depends upon its state of polarization. In general, when light rays enter such an anisotropic medium, they split up into two rays which have orthogonal polarization. They propagate along different directions. This phenomenon of one light ray giving rise to two refracted rays is called double refraction or birefringence. One of these rays is called an ordinary ray, while the other is called an extraordinary ray. The ordinary ray follows Snell's laws of refraction while the extraordinary ray does not. The ordinary ray has the same velocity along all directions of propagation, while the the velocity of the extraordinary ray depends upon the direction of propagation. There can be one or two directions in the crystal in which the velocities of the ordinary and the extraordinary are the same. If there is only one such direction, the crystal medium is called an uniaxial medium. If there are two such directions, it is called a biaxial medium. Quartz is an example of an uniaxial medium, while borax is a biaxial medium. This phenomenon is useful in making polarization devices such as wave plates, and in non-linear optical applications such as second harmonic generation, sub-harmonic generation and parametric amplification.


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