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 Light passing through lower polar, plane polarized, encounters sample and is split into fast and slow rays. When the light reaches the upper polar, a component of each ray is resolved into the vibration direction of the upper polar. 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.
Result is no light passes the upper polar and the grain appears black.
Both components are in the same direction, so the light constructively interferes and passes the upper polar.