Past exam of the physics A course of the University of Cambridge / 2011 / Paper 1 / Section A / 3 / Solution

For source and screen distances $s,r$, the paraxial optical path excess at aperture radius $u$ is $u^2(1/s+1/r)/2$. The outer boundary of Fresnel half-period zone $n$ has excess $n\lambda /2$, henceThis is the outer radius of the second zone, measured from the axis.