Q.How will polaroid be used to distinguish between unpolarised light and linearly polarised light?

Rotate a single polaroid in the beam through 360°: constant brightness means unpolarised light; brightness that varies and drops to zero twice per revolution means linearly polarised light.

Method: Look at the light through the polaroid and rotate the polaroid slowly about the direction of the beam.

Case 1 - Unpolarised light: it contains vibrations in all directions perpendicular to propagation equally. The polaroid always passes the same average fraction (half) of the intensity, so the transmitted brightness does NOT change as the polaroid is rotated - it stays constant.

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