(a) Why does unpolarised light from a source show a variation in intensity when viewed through a polaroid which is rotated? Show with the help of a diagram, how unpolarised light from sun gets linearly polarised by scattering.

(b) Three identical polaroid sheets  are oriented so that the pass axis of ${\mathrm{P}}_{2}$ and ${\mathrm{P}}_{3}$ are inclined at angles $60°$ and $90°$ respectively with the pass axis of ${\mathrm{P}}_{1}$.

A monochromatic source S of unpolarized light of intensity ${\mathrm{I}}_{0}$ is kept in front of the polaroid sheet ${\mathrm{P}}_{1}$ as shown in the figure.

Determine the intensities of light as observed by the observer at O, when Polaroid ${\mathrm{P}}_{3}$ is rotated with respect to ${\mathrm{P}}_{2}$ at angles $\mathrm{\theta }=30°$ and $60°$