How are Fresnel zones built?

Let us denote the distance from point O to the nearest point of the wave surface B through r0. The first Fresnel zone is limited by points on the wave surface, the distance from which to point O is equal to r1 = rо + n / 2. These points are located on a circle. The second zone is located between the edge of the first zone and the points of the wave surface, the distance from which to point O is equal to r2 = r1 + n / 2 = r0 + n. Similarly, we get r3 = r2 + k / 2 = r0 + 3k / 2, etc. …