• Tags: huygens, wavefront, basics
• Difficulty: Foundation

Huygens' principle states that every point on a wavefront serves as a source of secondary spherical wavelets. The new wavefront at a later time is the forward envelope of all secondary wavelets. A point source produces spherical wavefronts; a distant source produces plane wavefronts. Huygens' principle successfully explains reflection, refraction, and diffraction without requiring the concept of light as particles. It cannot, however, explain the photoelectric effect or Compton effect (which require quantum theory). The principle predicts that light slows down in a denser medium (v = $\frac{c}{n}$), consistent with the wave theory — unlike Newton's corpuscular theory which predicted faster light in denser media.