(a) Show, giving via suitable diagram, how unpolarized light can be polarised by reflection. (b) Two polaroids $P_1$ and $P_2$ are placed with their pass axes perpendicular to each other. Unpolarised light of intensity $I_0$ is incident on $P_1$. A third polaroid $P_3$ is kept in between $P_1$ and $P_2$ such that its pass axis makes an angle of $60^{\circ}$ with that of $P_1$. Determine the intensity of light transmitted through $P_1, P_2$ and $P_3$.