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 $45^{\circ}$ relative to that of A. The intensity of the emergent light is :

Question

$\begin{array}{1 1} (1)\;I_0 \\ (2)\;\large\frac{I_0}{2} \\(3)\;\large\frac{I_0}{4}\\ (4)\;\large\frac{I_0}{8} \end {array}$