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

As per the given question, monochromatic light emerging from polaroid (I) is plane polarized. When polaroid (II) is placed in front of this polaroid (I), and rotated till no light passes through polaroid (II), then (I) and (II) are set in crossed positions, i.e., pass axes of I and II are at `90^(@)`.
When a third polaroid (III) is placed inbetween (I) and (II). In all other cases, light will emerge from (II), as pass axis of (II) will no longer be at `90^(@)` to the pass axis of (III).