Two Polaroids $P_1$, and $P_2$ are placed with their axis
perpendicular to each other. Unpolarised light $I_o$ is
incident on $P_1$. A third polaroid $P_3$, is kept in
-between $P_1$, and $P_2$, such that its axis makes an angle
$45^{\circ}$ with that of $P_1$. The intensity of transmitted Iight
through $P_2$, is:
