A monochromatic source is placed in front of polaroid (I), and polaroid (II) is positioned in front of polaroid (I) (I). The light is unaltered as it travels through the polaroid (II).
The Polaroid (II) is turned such that no light passes through. The polaroid (II) is rotated such that it is perpendicular to the polaroid (I) (I). There is no light passage when the polaroid (II) is turned.
When a polaroid (III) is placed between the polaroid (I) and (II) such that the polaroid (III) is not placed perpendicular to the axis, so there is no passage of light.