INTERFERENCE AT THE UPPER POLAR

Now look at the interference of the fast and slow rays after they have exited the anisotropic mineral.

fast ray is ahead of the slow ray by some amount = D

Interference phenomena are produced when the two rays are resolved into the vibration direction of the upper polar.

Interference at the Upper Polar - Case 1
  1. Light passing through lower polar, plane polarized, encounters sample and is split into fast and slow rays.
  2. If the retardation of the slow ray = 1 whole wavelength, the two waves are IN PHASE.
  3. When the light reaches the upper polar, a component of each ray is resolved into the vibration direction of the upper polar.
  4. Because the two rays are in phase, and at right angles to each other, the resolved components are in opposite directions and destructively interfere and cancel each other.
  5. Result is no light passes the upper polar and the grain appears black.
Interference at the Upper Polar - Case 2
  1. If retardation of the slow ray behind the fast ray = ½ a wavelength, the two rays are OUT OF PHASE, and can be resolved into the vibration direction of the upper polar.
  2. Both components are in the same direction, so the light constructively interferes and passes the upper polar.