Science Puzzle
Why Does the Straw Look Bent?
Drop a straw into a glass of water and look at it from the side. Above the waterline the straw is perfectly straight. Below the waterline it appears to kink sharply at the surface, as though someone had bent it.
Lift the straw out and it is straight again. Nothing ever happened to the straw. So what exactly is your eye seeing?
The Answer
The straw never bends. The light does.
Light travels more slowly through water than through air, roughly a quarter slower. When light from the underwater part of the straw crosses the boundary between water and air, the change in speed makes it change direction. This is called refraction.
Your brain, however, assumes that light always travels in straight lines. It traces each ray of light straight back into the glass and places the underwater part of the straw where the light appears to have come from, not where the straw actually is. The result is a straw that looks broken at exactly the waterline, the one place where the light changes speed.
The principle: Refraction. Light changes speed, and therefore direction, when it passes from one transparent material into another. The same effect makes swimming pools look shallower than they are and lets lenses focus light in glasses, cameras and your own eyes.