Birefringence, difference between the index of refraction of the slow and fast rays and the interference colours for uniaxial minerals is dependant on the direction that light travels through the mineral.

  1. In a sample which has been cut perpendicular to the optic axis, the bottom and top surfaces will be parallel. The angle of incidence for the light entering the crystal = 0° and the wave front are not refracted at the interface and remain parallel to the mineral surface.

  2. Cutting the sample such that the optic axis is parallel to the surface of the section the following is observed.

  3. A mineral cut in a random orientation, with normally incident light;