A plane wave $($wavelength $)$ of intensity $I_0$ is propagating in the z$-$direction. A large cardboard is placed on the $x-y$ plane to block all the light in the lower$-$half plane At a distance $z$ away from the cardboard, we would observe the light distribution along y$-$direction to be like the left figure below. Use Fresnel diffraction formula and the Cornu Spiral provided below to find the location of $P$ on the $y-$axis. $(12\%)$