implying that permitivity is independent of the direction. For anisotropic medium, in the simplest possible case,

             ,

              and

             .

From this, one can say that the refractive indicies are different in different directions for anisotropic crystals and hence the speed of propagation of light in such crystals will depend on the direction of propagation and the polarization of light. These crystals are called as birefringent crystals or doubly refracting medium.

When an unpolarized light (which is equivalent to the two orthogonally polarized light) falls on such crystals, different polarization will propagate through the crystal with different speed and it will result into elliptical wave front as shown in Fig 2.