Why Does a Straw Look Bent?

Step 1 of 5

Light travels slower in water than in air.

Water molecules get in the way, dropping light from 300,000 to about 225,000 km per second. Glass slows it more still.

Step 2 of 5

Hitting a boundary at an angle makes it turn.

If a beam meets the surface slanted, one side crosses first and slows first. The whole beam pivots — exactly like a trolley with one wheel hitting sand.

Step 3 of 5

Your brain refuses to believe it.

Your visual system always assumes light arrived in a straight line. It traces the bent rays back along a straight path and puts the underwater part of the straw in the wrong place.

Step 4 of 5

It is why water looks shallower than it is.

The same bending makes the bottom of a pool appear about a quarter closer than it really is. Spear fishers learn to aim below the fish.

Step 5 of 5

Push the angle far enough and light cannot get out at all.

Beyond a certain angle, light hitting the underside of the surface reflects back down instead of escaping. Fibre optic cables use this to trap light for thousands of kilometres.

Refraction is also why the setting Sun is visible after it has actually dropped below the horizon.