Huygens' wave theory explains light using wavefronts and wavelets

Turning Points in Physics | AQA A-Level Physics

Key Definition

Huygens' principle: Every point on a wavefront acts as a secondary source of circular wavelets. The new wavefront is the envelope (tangent surface) of all these wavelets.

What the wave theory explains

Reflection: When a wavefront hits a surface, each point on the surface becomes a new source of wavelets. The envelope of these wavelets produces a reflected wavefront, with the angle of incidence equal to the angle of reflection.
Refraction: When a wavefront enters a denser medium, it slows down. The part of the wavefront that enters first slows first, causing the wavefront to change direction towards the normal. The key part is that this model predicts light is slower in a denser medium.
Diffraction: When a wavefront passes through a narrow gap, wavelets from the gap spread out in all directions. This naturally explains how light bends around obstacles and spreads through slits.

Comparing the two theories

Both theories explain reflection and refraction, but they make opposite predictions about the speed of light in a denser medium.
Newton (corpuscular): light is faster in a denser medium.
Huygens (wave): light is slower in a denser medium.
In 1850, Foucault measured the speed of light in water and found it was slower than in air. This decisively supported the wave theory.
However, the wave theory could not explain the photoelectric effect (discovered later), which required a particle model. This eventually led to the modern concept of wave-particle duality.

Common Mistake

Do not say "the wave theory was proven correct and the particle theory was wrong." Both have limitations. The photoelectric effect (1905) required a particle explanation. Modern physics uses wave-particle duality: light behaves as a wave in some experiments and as a particle in others.