What is Snell's law of refraction?

Snell's law: n₁ sin θ₁ = n₂ sin θ₂. Light changes speed when it passes from one material into another, and that makes it bend. Every material slows light by a factor n, its refractive index: v = c ÷ n.

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Chapters in this interactive model

1. Light bends when it changes speed: Shine a laser from one material into another and watch it turn: n₁ sin θ₁ = n₂ sin θ₂.
2. Bent straws, shallow pools and rainbows: Refraction fools your eyes at every glass of water, and splits sunlight into colours.
3. A lens is Snell’s law, twice: Curve the glass and every ray bends by just the right amount to meet at one point.
4. Total internal reflection: light that can’t get out: Past the critical angle a surface becomes a perfect mirror. The internet, diamonds and binoculars all use it.
5. Mirages, backwards bending, and why light really slows: Snell’s law bends in hot air, breaks the rules in metamaterials, and hides a common myth.