Describe briefly how an unpolarized light gets linearly polarized when it passes through a polaroid.
(b) Three identical polaroid sheets ${\mathit{P}}_1,{\mathit{P}}_2$ and ${\mathit{P}}_3$ are oriented so that the pass axis of ${\mathit{P}}_2$ and ${\mathit{P}}_3$ are inclined at angles of ${60}^{\circ }$ and ${90}^{\circ }$ respectively with respect to the pass axis of ${\mathit{P}}_1$ . A monochromatic source $\mathit{S}$ of unpolarizedlight of intensity ${\mathit{I}}_0$ is kept in front of the polaroid sheet ${\mathit{P}}_1$ as shown in the figure. Determine the intensities of light as observed by the observers ${\mathrm{O}}_1,{\mathrm{O}}_2$ and ${\mathrm{O}}_3$ as shown.

OR
(a) Use Huygens geometrical construction to show how a plane wave-front at $\mathit{t}=0$ propagates and produces a wave-front at a later time.
(b) Verify, using Huygens principle, Snell's law of refraction of a plane wave propagating from a denser to a rarer medium.
(c) When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency. Explain why.