Thus for a given grain in a particular section, wavelength is the only variable and the phase difference will vary for the different wavelengths in the white light supplied. Hence certain colours will be reinforced because of path differences which happen to coincide with a whole range of wavelengths, and others will be cut out because of phase differences involving a half wavelength.
The result is a coloured ray, i.e. a coloured appearance of the grain (as seen on the left), due tothe removal of certain wavelengths from the original white. For a more rigorous discussion of interference colours see the Optical Mineralogy module - however, do not worry if you find this material challenging.
Birefringence