Huygens' Principle
Also known as: Huygens-Fresnel Principle · Wavelet Construction
Every point a wave reaches becomes the launch point of a new little wave. Add all those wavelets up and their leading edge is where the wave goes next. Where speed changes, the wavelets shrink and the front tilts — that bend is refraction.
A straight wavefront advances; each point spawns an expanding circular wavelet, and their common envelope forms the next wavefront.
Equivalent forms
One geometric rule — each point is a new source — quietly contains reflection, refraction, and diffraction all at once.
Unit systems
- SI:
- v in m/s, t in s, r in m
- CGS:
- v in cm/s, t in s, r in cm
- Imperial:
- v in ft/s, r in ft
Where it holds
Huygens presented his wavelet construction to the Paris Academy in 1678 (published 1690 in Traite de la Lumiere) to explain reflection and refraction without Newton's corpuscles. Fresnel revived and completed it in 1818 by adding interference between the wavelets, finally explaining diffraction.
How can light bend around a corner if it travels in straight lines?
Treat every point on a wavefront as a tiny source of new spherical wavelets. Their common envelope is the next wavefront — and from that single idea fall reflection, refraction, and diffraction.