Polaroids can decrease the intensity of light.

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 _____$\mathrm{W} {\mathrm{m}}^{-2}$.

Unpolarised red light is incident on the surface of a lake at incident angle ${\theta }_R$. An observer seeing the light reflected from the water surface through a polariser notices that on rotating the polariser, the intensity of light drops to zero at a certain orientation. The red light is replaced by unpolarised blue light. The observer sees the same effect with reflected blue light at incident angle ${\theta }_B$.

Then,

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

A transparent medium shows relation $i$ and $r$ as shown. If the speed of light in vacuum is $c,$ the Brewster angle for the medium is