Grade 11 · Physics
Refraction & Snell’s Law
Why it matters
Bending light and total internal reflection.
Last reviewed 2026-09-03 · accuracy standard
What is refraction and Snell’s law?
Short answer
Refraction is the bending of light as it passes from one medium into another because its speed changes. Snell’s law quantifies it: n₁ sin θ₁ = n₂ sin θ₂, where n is each medium’s refractive index and θ the angle from the normal. Light entering a denser medium bends toward the normal; beyond a critical angle it undergoes total internal reflection.
Concept preview
What you will be able to do
- 01Explain refraction using the change in the speed of light
- 02Use the refractive index and Snell's law (n₁sinθ₁ = n₂sinθ₂)
- 03Explain total internal reflection and the critical angle
Light travels fastest in a vacuum (and nearly as fast in air), but slows down inside glass or water. When a ray crosses into a new medium at an angle, one side of the beam slows before the other, so the whole ray bends — this is refraction. It's the reason a straw looks snapped at the water line and a pool looks shallower than it really is.
School Plan
Refraction & Snell’s Law
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Related topics
Frequently asked questions
- Why does light bend when it enters a new medium?
- Its speed changes; the change in speed at the boundary alters its direction, which we see as bending.
- What is total internal reflection?
- When light in a denser medium hits the boundary beyond the critical angle, it reflects entirely instead of refracting out.