A straw in a glass of water looks bent at the surface, and a pool looks shallower than it is. Both happen because light transmitted into a new material changes speed, and that can make it change direction. This bending is called refraction.
Key idea
Light bends when it crosses a boundary at an angle and changes speed. Going into a slower material (air into water or glass), it bends toward the normal (the line perpendicular to the surface). Hitting the boundary straight on, it slows down but does not bend.
Why it works
Picture a marching band crossing from pavement onto mud at an angle. The marchers who reach the mud first slow down first, while the others keep going, so the whole line swings around and heads in a new direction. If the band reaches the mud square on, every marcher slows at the same moment, and the line keeps going straight. Light's wavefronts do the same thing.
Worked example: Into water
A ray of light goes from air into water, hitting the surface at 30° from the normal. Does it bend toward or away from the normal?
- Light travels slower in water than in air.
- Going into a slower material, it bends toward the normal.
Answer: Toward the normal
A lens uses refraction on purpose. A converging lens, thicker in the middle, bends parallel rays inward so they meet at a point, the focal point. That is how a magnifying glass, a camera and the lens of your eye form images.
Common mistake
Expecting light to bend even when it meets a boundary straight on. Along the normal, the light slows down but keeps its direction. Bending needs an angle, so that one side of the beam slows before the other.
Bend laser beams through glass and water on the Optics Bench.