Huygens' Principle

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.