Light bends because
one edge slows first
The wavefronts tell the story better than the ray does. As each front meets the denser medium, the leading edge slows while the trailing edge is still going fast, and the whole front pivots. Push the angle far enough and nothing crosses at all.
whatA beam meeting a boundary between two media, drawn as rays and as wavefronts. The front spacing is the wavelength inside each medium.
lawn₁sinθ₁ = n₂sinθ₂. Frequency is fixed, so a slower medium means shorter wavelength — which is what pivots the front.
trySwap the media so the light goes from dense to sparse, then raise the incidence angle.
noticePast the critical angle Snell's law has no solution: refraction disappears and everything reflects back.
in lifeOptical fibre carries light for kilometres by keeping every ray past the critical angle.
found byIbn Sahl had the law in 984, six centuries before Snell and Descartes; Fermat later derived it from least time.