A source of light is placed in front of a screen. Intensity of light on the screen is $I$. Two Polaroids $P_1$ and $P_2$ are so placed in between the source of light and screen that the intensity of light on screen is $\mathrm{I} / 2 . \mathrm{P}_2$ should be rotated by an angle of $\_\_\_\_$ (degrees) so that the intensity of light on the screen becomes $\frac{3 \mathrm{I}}{8}$.