Huygens' Principle
Each wavefront point emits secondary wavelets — envelope = new wavefront.
Key Notes
Huygens' Principle: every point on a wavefront acts as a source of secondary spherical wavelets. The new wavefront at a later instant is the forward envelope of these wavelets.
A wavefront is a surface of constant phase. Light travels perpendicular to the wavefront, in the direction of the ray.
The principle explains rectilinear propagation, reflection, and refraction without invoking corpuscles — and is consistent with Snell's law.
It works for any wave (sound, water, light) and is the wave-optics foundation of every phenomenon you'll meet in this chapter — interference, diffraction, polarization.
Newton's corpuscular theory could not explain interference and diffraction; Huygens' wave model can.
The 'obliquity factor' (1 + cos θ)/2 in the modern Fresnel-Kirchhoff form means wavelets are stronger in the forward direction — there is no backward wavefront, resolving an old objection to Huygens.
Formulas
Secondary wavelet
Amplitude of a spherical wavelet from a wavefront point.
New wavefront
Geometric construction of the propagated wavefront.
Snell from Huygens
Refraction derived by equating tangential wavefront speeds.
Wavefront speed
Phase speed of a wavefront in a medium of index n.
Important Points
Plane wavefront → plane (parallel rays). Spherical wavefront → diverging or converging rays. After a converging lens, an incoming plane front becomes a spherical front converging to the focus.
Huygens' construction predicts no backward wave — a feature accounted for by the obliquity factor.
Whenever a wave passes through a slit narrower than its wavelength, the wavefronts spread out as expanding spherical wavelets — that's diffraction.
Modern QM analogue: amplitude at a point is the sum of contributions from all paths (Feynman path integral) — Huygens at the quantum level.
Huygens' Principle notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.