Double Refraction

Double Refraction: When a ray of light passes through an an-isotropic crystal, it breaks into two polarized rays vibrating in mutually perpendicular planes. One ray which obeys laws of refraction is called ”ordinary ray” or ”O-ray”, and the second ray which does not obey these laws is called ”extraordinary ray” or ”E ray”.

• Ordinary Ray: The ordinary ray travels with the same velocity in all directions through the crystal. This ray will, therefore, have a constant refractive index.
• Extr-Ordinary Ray: The velocity of this ray changes with the variation of its path through the crystal and therefore, its refractive index also changes. 

• Birefringence: The difference between refractive indices of the O-ray and E-ray is called ”birefringence”. For biaxial crystals, the numerical difference between the greatest and least refractive indices is the birefringence.
• Optic Axis: A direction in an an-isotropic mineral, along which there is no double refraction, is called ”optic axis”. The minerals crystallizing in the tetragonal and hexagonal systems have one optic axis and therefore, they are called ”uniaxial”. The minerals belonging to the orthorhombic, monoclinic, and triclinic systems are called ”biaxial” because they have two optic axis.