Linked from
The 19 pages that link to Huygens–Fresnel principle, each with the reason it gives.
Christiaan HuygensBroader topic: It extends Huygens’s account of how light waves propagate.
ReflectionRelated: It describes reflected wavefronts as the boundary redirects secondary wavelets.
WavefrontRelated: It predicts a new wavefront from the current one.
Airy diskRelated: Interference among wavelets across the aperture forms the disk and its rings.