In the half-wave slit model, we define the family of curves passing through the intensity field extrema behind the slit as 'extremal' (or 'stationary') lines.
You will find images created using this applet in [1].
Location of Extremal Line Intersections
In the half-wave slit model, the near-field (Fresnel) diffraction field behind a slit (b = 6, λ = 1) is structured by a family of continuous extremal (or stationary) lines. The intersections of these lines correspond to critical points (local maxima, minima, or saddle points) in the intensity distribution J(x, y), which define the exact conditions for destructive and constructiveinterference of waves from half-wave zones.
