Describe briefly how an unpolarized light gets linearly polarized when it passes through a polaroid.(b) Three identical polaroid sheets $P_1, P_2$ and $P_3$ are oriented so that the pass axis of $P_2$ and $P_3$ are inclined at angles of $60^{\circ}$ and $90^{\circ}$ respectively with respect to the pass axis of $P_1$ . A monochromatic source $S$ of unpolarizedlight of intensity $I_0$ is kept in front of the polaroid sheet $P_1$ as shown in the figure. Determine the intensities of light as observed by the observers $O_1, O_2$ and $O_3$ as shown. OR (a) Use Huygens geometrical construction to show how a plane wave-front at $t=0$ 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.