In a Young's double slit experiment, three polarizers are kept as shown in the figure. The transmission axes of $P_{1}$ and $P_{2}$ are orthogonal to each other. The polarizer $P_{3}$ covers both the slits with its transmission axis at $45^{\circ}$ to those of $P_{1}$ and $P_{2}$. An unpolarized light of wavelength $\lambda$ and intensity $I_{0}$ is incident on $P_{1}$ and $P_{2}$. The intensity at a point after $P_{3}$ where the path difference between the light waves from $s_{1}$ and $s_{2}$ is $\frac{\lambda}{3}$, is