Problem 5.97

A plane light wave falls normally on a diaphragm with round a perture opening the first $N$ Fresnel zones for a point $P$ on a screen located at a distance $b$ from the diaphragm. The wavelength of light is equal to $\lambda .$ Find the intensity of light $I_{0}$ in front of the diaphragm if the distribution of intensity of light $I(r)$ on the screen is known. Here $r$ is the distance from the point $P$.