interference colour

                  INTERFERENCE COLOUR 

An interference colour is the colour observed for a mineral in thin-section under crosed polarized light. Interference colours can provide important clues to the identity of a mineral or even the orientation of the crystal in the section. Interference colours are produced because plane polarised light passing through a mineral section splits into two rays vibrating at 90 degrees to each other. The two rays travel at a different velocities depending on the refractive index of a mineral in the direction of the vibration. The rays are, therefore, called the fast ray and the slow ray. Because the fast and slow ray travel at different velocities they become out of phase and when they recombine at the analyser they interfere to produce light of a characteristic wavelength. Interference colours change with the orientation of the mineral section for anisotropic minerals because the refractive indices of the mineral are different in different directions. Isotropic minerals do not show interference colours (they are black in cross polarised light) since their refractive index is the same in every direction. 
 
 The interference colour of a section of a mineral can be predicted from the birefringence- the difference between the refractive indices in the vibration directions of the fast and slow ray. Usually optical mineralogy text books provide the range of principal refractive indices for the principal axes of the mineral, and the orientation of these axes relative to the crystallographic axes of the mineral. The interference colour produced also changes with the thickness of the section.