State clearly how an unpolarised light gets linearly polarised when passed through a polaroid. Unpolarised light of intensity I₀ is incident on a polaroid P₁ which is kept near another polaroid P₂ whose pass axis is parallel to that of P₁. How will the intensities of light, I₁ and I₂, transmitted by the polaroids P₁ and P₂ respectively, change on rotating P₁ without disturbing P₂?

Polaroid consists of long chain polymers which allow the vibration of electric field inside them in only one direction. Hence, the output light wave is linearly polarized in a single direction as shown in the attached figure (i)On passing light through a single polaroid, intensity is reduced to half. The output intensity from Polaroid 1 is given by: I₁=I₀/2 Hence, I₁ will not change on rotating P₁. However, intensity of I₂ will be changed as discussed below (ii)Initially, axis of Polaroid 2 is aligned to that of Polaroid 1 and then, Polaroid 1 is rotated so that angle between them changes to θ. Hence, from Malus' Law I₂=I₁cos²(θ) I₂=I₀/2cos²(θ)