Consider a light beam incident from air to a glass slab at Brewster’s angle as shown in Fig. 10.1. A Polaroid is placed in the path of the emergent ray at point P and rotated about an axis passing through the centre and perpendicular to the plane of the polaroid.
The intensity of light as seen through the Polaroid shall go through a minimum but not zero for two orientations of the Polaroid.
In the given question it is stated that the angle of incidence of unpolarized light is equal to Brewster’s angle, therefore polarization of reflected ray will happen, but emergent ray will still be unpolarised in nature. Therefore there will no particular orientation such that there shall be complete darkness while observing through polaroid. But still, the intensity of light will be dependent on the rotation of Polaroid, as the light ray which will be parallel to the axis of Polaroid will be able to pass through it. Therefore at a particular angle, the intensity of light will go minimum but not zero.