Before going on to examine how light inteacts with minerals we must define one term:
|RETARDATION - (delta) represents the distance that one ray lags behind another. Retardation is measured in nanometres, 1nm = 10-7cm, or the number of wavelengths by which a wave lags behind another light wave.|
The relationship between rays travelling along the same path and the interference between the rays is illustrated in the following three figures.
In a vacuum light travels at 3x1010 cm/sec (3x1017 nm/sec).
When light travels through any other medium it is slowed down, to maintain constant frequency the wavelength of light in the new medium must also changed.