<P> The case of so - called biaxial crystals is substantially more complex . These are characterized by three refractive indices corresponding to three principal axes of the crystal . For most ray directions, both polarizations would be classified as extraordinary rays but with different effective refractive indices . Being extraordinary waves, however, the direction of power flow is not identical to the direction of the wave vector in either case . </P> <P> The two refractive indices can be determined using the index ellipsoids for given directions of the polarization . Note that for biaxial crystals the index ellipsoid will not be an ellipsoid of revolution ("spheroid") but is described by three unequal principle refractive indices n, n and n . Thus there is no axis around which a rotation leaves the optical properties invariant (as there is with uniaxial crystals whose index ellipsoid is a spheroid). </P> <P> Although there is no axis of symmetry, there are two optical axes or binormals which are defined as directions along which light may propagate without birefringence, i.e., directions along which the wavelength is independent of polarization . For this reason, birefringent materials with three distinct refractive indices are called biaxial . Additionally, there are two distinct axes known as optical ray axes or biradials along which the group velocity of the light is independent of polarization . </P> <P> When an arbitrary beam of light strikes the surface of a birefringent material, the polarizations corresponding to the ordinary and extraordinary rays generally take somewhat different paths . Unpolarized light consists of equal amounts of energy in any two orthogonal polarizations, and even polarized light (except in special cases) will have some energy in each of these polarizations . According to Snell's law of refraction, the angle of refraction will be governed by the effective refractive index which is different between these two polarizations . This is clearly seen, for instance, in the Wollaston prism which is designed to separate incoming light into two linear polarizations using a birefringent material such as calcite . </P>

A prism is a transparent object that bends light. what is the figurative meaning of prism