Between the point source S and the observation point A, a disk is placed, the center of which is located on the same straight line with points S and A (figure). One half of the disk is transparent, the other is opaque. The disk covers the first three Fresnel zones. Thickness of the transparent part of the disk l = N, where n is the refractive index of the transparent part of the disk, N is an integer. What is the intensity at point A for even and odd N?

Short solution. Format gif