Ordinary glass is a good example of isotropic material. There are certain material for which the refractive index is different in different direction or we can say simply that the speed of propagation will not be same in all direction and also it may depend on the state of polarization. In such materials a point source may result into the elliptical wave warfront or for different polarization the wave front will be different, as shown in fig 1(b). 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. Recall that refractive index n is given by (module 13, lecture 1, eq 1.7)

                          

Which depends on permitivity or polarization property of the medium. The polarizibility of a medium is given by for isotropic medium,

                        ,