• $summary_{type}$: concept
• $word_{count}$: 150

Huygens' principle states that every point on a wavefront acts as a secondary source, and the new wavefront is the envelope of all secondary wavelets. This elegantly explains reflection (angle of incidence = angle of reflection), refraction (Snell's law with $v_{1}$/$v_{2}$ = $n_{2}$/$n_{1}$), and diffraction (bending around obstacles). A key prediction of wave theory is that light slows down in denser media (v = $\frac{c}{n}$), which was confirmed by Foucault's experiment — contradicting Newton's corpuscular theory. Huygens' principle fails to explain the photoelectric effect and polarization without additional wave properties (transverse nature). For JEE, understand that plane wavefronts come from distant sources, spherical wavefronts from point sources, and cylindrical wavefronts from line sources. The principle is primarily tested conceptually.