Light from an original single, plane polarised (white) beam passes through anisotropic crystals as two rays with different velocities.
When the analyser is inserted, the two rays are recombined into a single N-S plane and interfere with each other as they will be out of phase.
The phase difference (order) depends on:
- The thickness of the section
- The difference in refractive index between the two rays (constant for all wavelengths if no dispersion)-called the birefringence
- The wavelengths of the light
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.
We compare this colour with interference colour chart reading the orders.
||Interference colours are 1st order, grey to white. Gypsum (PX)
||Interference colours are 2nd order Olivine (PX)
||Interference colours are 3rd order Calcite (PX)