The expression for intensity (as shown in the figure) of transmitted light when a polaroid sheet is rotated between two crossed polaroids is:
[Given: \(I_0\) is the intensity of transmitted light from polariser \(\mathrm{A}\)]

Unpolarised light is incident on a plane glass surface. What should be the angle of incidence so that the reflected and refracted rays are perpendicular to each other?

$1. {45}^0$
$2. {67}^0$
$3. {57}^0$
$4. {37}^0$