Ordinary glass is a great illustration of isotropic material. There are a number of material for which the refractive index is different in different direction or we can say simply that the speed of propagation will never be same in all direction as well as it may well rely upon the state of polarization. In such materials a point source may result into the elliptical wave warfront or for different polarization the wave front is going to be different, as shown in fig 1. Such materials are called as anisotropic material. Crystals like Quartz (SiO 2), calcite (CaCo 3) and KDP (potassium dihydrogen phosphate, KH 2 PO 4) are good example of anisotropic material. Such anisotropy is due to difference in the arrangement of the atoms and molecules in different direction in the crystal which leads to direction dependent polarization property of the system