Two Polaroids $P_1$ and $P_2$ are placed with their axis perpendicular to each other. Unpolarised light $l_0$ is incident on $P_1$. A third polaroid $P_3$ is kept in between $P_1$ and $P_2$ such that its axis makes an angle $45^\circ$ with that of $P_1$. The intensity of transmitted light through $P_2$ is