Huygens' Principle
Simple Explanation
Huygens' principle states that every point on a wavefront acts as a source of tiny secondary wavelets, spreading outward at the wave's own speed — the new wavefront a moment later is simply the envelope (the common tangent surface) that all those wavelets together sweep out.
Why Do We Need It?
This deceptively simple idea explains how waves propagate, bend around obstacles, and produce interference and diffraction patterns — it is the conceptual foundation for everything else in this chapter.
See It
An inner curved arc representing the old wavefront, with several small dashed circles centered along it representing secondary wavelets, and an outer curved arc representing the new wavefront formed by their combined envelope
Worked Example
Use Huygens' principle to explain wave bending at a barrier edge
Explain, using Huygens' principle, why a wave passing through a narrow gap spreads out into the space behind the barrier, instead of continuing in a single straight beam.
Why Does This Work?
Deep inside a wide wavefront, each secondary wavelet's sideways spreading is almost perfectly canceled by the wavelets right next to it, leaving only the forward-moving wavefront — but at an edge (like the boundary of a narrow gap), there is no wavelet on one side to cancel the spreading, so the wave visibly bends into the shadow region.
Real-Life Example
Hearing around a corner
A person can hear someone talking around a corner, even without a direct line of sight to them.
Sound waves diffract (spread) around the corner's edge, exactly as Huygens' principle predicts — each point along the wavefront at the corner acts as a new source, sending sound into the region that would otherwise be a 'shadow.'
Practice
According to Huygens' principle, what does every point on a wavefront act as?
MediumCommon mistake
Thinking diffraction (wave-spreading) only happens for sound and never for light — light diffracts too, just far less noticeably in everyday life because its wavelength is so much smaller than most everyday openings.
Quick Review
- Every point on a wavefront is a source of secondary wavelets.
- The new wavefront is the envelope (common tangent) of those wavelets.
- Explains why waves bend around obstacles and through narrow gaps.