Reflection and refraction: how light bends
Light bounces off surfaces and bends through them by two simple rules. Get the normal line right and you'll never muddle the angles again — including why a straw looks broken in water.
Light does two things when it meets a new surface: some bounces off (reflection) and some passes through, bending as it goes (refraction). Both are measured the same way — from an imaginary line called the normal, drawn at right angles to the surface.
Reflection: equal angles
For any mirror, the angle the light comes in at equals the angle it leaves at, both measured from the normal:
A flat mirror gives an image that is the same size, upright, and laterally inverted (left and right swapped) — which is why text looks back-to-front in a mirror.
Refraction: bending at the boundary
When light passes from one material into another (air into glass, say), it changes speed, and that change of speed bends it. The rule for which way:
- Into a denser material (air → glass, light slows down) → it bends towards the normal.
- Into a less dense material (glass → air, light speeds up) → it bends away from the normal.
This is why a straw in a glass of water looks bent or broken at the surface: light from the underwater part bends as it leaves the water, so your eye is misled about where the straw is.
A quick mental model
Picture a car driving off a smooth road onto mud at an angle. The wheel that hits the mud first slows down while the other keeps going, so the car turns. Light does the same thing when half of a wave front enters the slower material first.
Draw the normal as a dotted line at to the surface, then measure every angle from it. Students who measure from the surface itself get angles that are out — the single most common refraction mistake.
Last revised 22 July 2025.