The orthogonality of the wavefronts to rays (or equal-action surfaces to trajectories) means we can compute one set from the other set. This explains how Kirchhoff's diffraction formula predicts a wave phenomenon – diffraction – using only geometrical ray tracing. Rays traced from the source to an aperture give a wavefront that becomes sources for rays reaching the diffraction pattern where they are summed using complex phases from the orthogonal wavefronts.