A source of light is placed in front of a screen. The 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 the screen is $\frac{I}{2}$. Then the $P_2$ should be rotated by an angle of (degrees) so that the intensity of light on the screen becomes $\frac{3I}{8}$.