A straw looks bent in water because light changes speed when it leaves the liquid.

The longer version

Light slows down when it enters water and speeds up again on leaving, and that change bends its path. The bending happens at the boundary, so rays coming from the submerged part of a straw reach the eye from a shifted direction. The brain assumes light always travels in straight lines and rebuilds the straw accordingly.

The rebuilt position is displaced, making the object appear kinked or broken at the surface. The same effect makes a pool look shallower than it is and makes fish appear slightly offset. How much bending occurs depends on the angle and on how greatly the two materials differ in speed.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.