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.

  1. If retardation is a whole number (i.e., 0, 1, 2, 3, etc.) of wavelengths.
    The two waves, A and B, are IN PHASE, and they constructively interfere with each other. The resultant wave (R) is the sum of wave A and B.

  2. When retardation is = ½, 1½, 2½ . . . wavelengths.
    The two waves are OUT OF PHASE they destructively interfere, cancelling each other out, producing the resultant wave (R), which has no amplitude or wavelength.

  3. If the retardation is an intermediate value, the the two waves will:
    1. be partially in phase, with the interference being partially constructive
    2. be partially out of phase, partially destructive.

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.