The Huygens Construction

The Huygens-Fresnel principle made physical: a wavefront is replaced by N secondary point sources, and their coherent superposition reconstructs the next wavefront and the diffraction pattern. The primary scene is the physical 2D wavelet field with the swept Huygens circles and the reconstructed wavefront; the side panel compares the N-source far field with the analytic uniform-aperture sinc envelope. A flat aperture reproduces single-slit Fraunhofer diffraction with intensity zeros at sin theta = m lambda / a, and a concave arc of equal-phase wavelets converges to a focus, showing that focusing and diffraction are the same interference physics. Drag the source count and aperture to see the construction build the pattern.