how can one distinguish between an unpolarised light beam and a linearly polarised light beam using a polaroid ?

Explanation

To distinguish between an unpolarised light and a linearly (plane) polarised light using a polaroid, place a polaroid in the path of given light. Slowly rotate the polaroid and observe the intensity of light transmitted through the polaroid. If the intensity of light varies and in one complete rotation of polaroid, two times light intensity is maximum and two times light intensity is zero i.e., it is complete darkness, then the given light is plane polarised light. on the other hand, if on rotation of polaroid the intensity of transmitted light remains unchanged, the given light is unpolarised light.