Refraction: why light bends in water and glass

0:00Why does a straight pencil look broken when you dip it in water?

0:05To find out, follow one ray of light as it passes from air into water.

0:11Where it hits, draw the normal: an imaginary line at right angles to the surface.

0:17In the water, the ray does not go straight on. It bends toward the normal.

0:23This bending as light passes from one material into another is called refraction.

0:30It happens because light slows down in water, and in glass too.

0:35In air, light travels at about 300,000 kilometres per second, in water about 225,000, and in glass about 200,000.

0:47Picture a cart rolling at an angle from smooth pavement onto sand.

0:53One wheel reaches the sand first and slows down, so the cart turns toward the normal.

0:59Straight on, both wheels slow at once and the cart goes straight. Light along the normal slows but does not bend.

1:09Light from a stone on the bottom speeds up as it leaves the water, and bends away from the normal.

1:16Your brain assumes light travels in straight lines, so it traces the ray straight back.

1:22So the bottom looks higher than it really is, and the pool looks shallower.

1:27The pencil works the same way: its underwater part looks raised, so it looks broken.

1:34But the pencil does not really bend. Only the light coming from it bends.

1:40So refraction is light bending as it crosses into a new material at an angle, because its speed changes.