Note:
In uniaxial crystals the optic axis coincides with the symmetry axis. The refractive index of the crystal is the same in all directions perpendicular to the optic axis but is different along the optic axis. Consequently, electromagnetic rays or waves that do not travel along the optic axis experience different refractive indices in different directions perpendicular to their direction of propagation. Two waves result, one has its electric vector perpendicular to the optic axis and forms the ordinary wave. The other is the extraordinary wave.