Unpolarised light of intensity $I_0$ falls on three polaroids P1, P2 and P3 in a row, all facing the beam as the figure shows. A polaroid passes only light polarised along its transmission axis. The transmission axes of P1, P2 and P3 make angles of $0^\circ$, $30^\circ$ and $90^\circ$ with the vertical. What is the intensity of the light that comes out of P3?

Fig. 1: Top row: a side view of a beam of unpolarised light of intensity I0 travelling to the right through three thin polaroid sheets in a row, P1, P2 and P3. Bottom row: the same three polaroids seen face on as circles, each with a double headed line for its transmission axis. The axis of P1 is vertical, the axis of P2 is turned 30 degrees from the vertical, and the axis of P3 is horizontal, 90 degrees from the vertical.