A polariod (I) is placed in front of a monochromatic source. Another polariod (II) is placed in front of this polariod (I) and rotated till no light passes. A third polariod (III) is now placed in between (I) and (II), then

Explanation

Monochromatic unpolarised light incident on the polaroid (I) will emerge out as plane polarised. When another polaroid (II) is placed in front of polaroid (I) such that their axes are inclined at`90^@` to each other , no light will be transmitted via polaroid (II).
A third polaroid (III) is now placed in between the polaroids (I) and (II). Only when the pass axis of polaroid (III) is parallel to the pass axis of polaroid (I) or (II), no light will emerge out of polaroid (II). In all other cases, light will emerge from polaroid (II), as pass axis of the polaroid (II) will no longer be at `90^@` to the pass axis of the polaroid (III).