Monochromatic plane waves perpendicularly illuminate a small circular hole in a screen. From point-P, beyond the hole on the central axis, exactly 3 Fresnel zones appear to fill the hole. If the incident irradiance on the aperture screen is I_u, prove that the irradiance at P is very nearly 4I_u. Because these are plane waves, the unobstructed irradiance at P would equal I_u.