An isotropic body will have the same refractive index for all the direction of incident light. Many crystals are said to be anisotropic that is birefringent. The two refracted waves emerging from a birefringent crystal are called the ordinary wave and the extraordinary wave. Each wave is polarized and vibrates in a plane different by 90 degrees to the other. Between crossed polarizing filters, birefringent crystals are visible.
Many birefringent crystals produce a characteristic interference pattern.
Uses Polarized light microscopy is a must for the identification of oval fat bodies. These contain lipid droplets containing cholesterol esters in a liquid crystal state witch shows an easily recognizable maltese cross interference pattern. Polarized light microscopy can also help for the identification of crystals. An uncolored and transparent object of refractive index n with a thickness e in an uncolored and transparent media of refractive index n' will be invisible unless the borders exhibit diffraction. Since the light rays passing through the object follow a different path than the light rays passing only in the media, the object is said to have a phase difference.