A beam of unpolarised light of intensity $I_0$ is passed through a polaroid $A$ and then through another polaroid $B$ which is oriented so that its principal plane makes an angle of $45°$ relative to that of $A$. The intensity of emergent light is :

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,

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