Consider a light beam incident from air to a glass slab at Brewster's angle as shown in figure.
A polaroid is placed in the path of the emergent ray at point P and rotated about an axis passing through the centre and pependicular to the plane of the plaroid.

Options

A. For a particular orientation, there shall be darkness as observed thorugh the polaroid.
B. The intensity of light as seen thorugh the polaroid shall be independient of the rotation.
C. The intensity of light as seen through the polaroid shall go through a minimum but not zero for two orientations of the polaroid.
D. The intensity of light as seen through the polaroid shall go through a minimum for four orientations of the polaroid.

Answer

C

Explanation

From Brewster.s law, the light reflected from the top of the glass slab is polarised. The light that is refracted into the glass slab and emerges out of it is only partially polarised. Partially polarised means that other component is not completely absent. Whenever axis of the polaroid gets aligned with this, then intensity will be minimum but not zero. If a polaroid is held in the path of the emergent ray from point P and is rotated about an axis passing through the centre and perpendicular to the plane of the polaroid, the intensity of transmitted light shall go through a minimum but not zero for two orientations of the polaroid. When crystal is rotated, then axis gets aligned twice in one full rotation.