Unpolarised light of intensity $32 \mathrm{W} {\mathrm{m}}^{-2}$ passes through the combination of three polaroids such that the pass axis of the last polaroids is perpendicular to that of the pass axis of first polaroids. If intensity of emerging light is $3 \mathrm{W} {\mathrm{m}}^{-2}$, then the angle between pass axis of first two polaroids is _________$°$.

As shown in figures, three identical polaroids $P_1$, $P_2$ and $P_3$ are placed one after another. The pass axis of $P_2$ and $P_3$ are inclined at angle of $60º$ and $90º$ with respect to axis of $P_1$. The source $S$ has an intensity of $256 \mathrm{W} {\mathrm{m}}^{-2}$. The intensity of light at point $O$ is _____$\mathrm{W} {\mathrm{m}}^{-2}$.

How will you distinguish experimentally between ordinary light and plane-polarized light?