Light refracts when it changes material — Physics, 11–13
Light can change direction when it enters glass, water or another material because its speed changes.
Light changes direction
When light crosses from air into water or glass, it usually changes direction. This bending is called refraction and happens because light travels at a different speed in the new material. The light has not stopped or bounced back; it has continued along a new path.
Why does a straw look bent?
People needed to understand why objects seen through water appear in the wrong place and how lenses can make useful images. Refraction solves this problem by tracking what happens when light crosses a boundary. A lens uses several curved surfaces to bend light in a controlled way.
The coin under the bowl
Put a coin on a table and step back until the rim of a bowl just hides it. Keep your head still, then ask someone to pour in water without moving the coin or bowl. The coin becomes visible because light from it bends at the water–air boundary before reaching your eyes. The coin did not move; its apparent position changed.
Refraction is not reflection
It is reasonable to think that a pencil looks bent in water because the water acts like a shiny mirror. A mirror sends light back from the same surface, which is reflection. Refraction happens when light goes through a boundary and changes direction, so the pencil can look displaced or bent.
Lenses that form images
Refraction is used in glasses, cameras, magnifying glasses, microscopes and telescopes. Curved lenses bend light so that it spreads out, comes together or focuses on a screen or the retina. Different lens shapes correct different focusing problems, but a lens cannot create detail that no light has carried.
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