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.
   • A cut through the indicatrix, parallel to the bottom of the mineral, will yield the indices and vibration directions of the light. A slice through the indicatrix is a circular section, with radius n_omega.
   • No preferred vibration direction, so light passes along the optic axis as an ordinary ray and retains whatever vibration direction it had originally.
   • Between crossed polars the light passing through the mineral is completely absorbed by the upper polar and will remain black on rotation of the stage, The birefringence = 0.

   

2. Cutting the sample such that the optic axis is parallel to the surface of the section the following is observed.
   • The indicatrix section is a principle section, as it contains the optic axis. The indicatrix forms an ellipse with axes = n_omega and n_epsilon, with the incident light being split into two rays such that:
     • the ordinary ray vibrates perpendicular to the optic axis,
     • the extraordinary ray vibrates parallel to the optic axis.
   • The birefringence is at a maximum, and in thin section this grain orientation will display the highest interference colour.

   

3. A mineral cut in a random orientation, with normally incident light;
   • The ordinary ray produced has an index, n_omega and vibrates perpendicular to the optic axis.
   • The extraordinary ray has an index n_epsilon' and vibrates in the plane containing the optic axis.
   • n_epsilon' < n_omega maximum or minimum, the birefringence is intermediate between the two extremes.