Why does a straw look bent in water?
Short answer
Light changes direction at the boundary between water and air, so the part of the straw under water appears shifted. The straw itself has not bent.
How it works
Light travels a little differently through air and water. When light changes direction as it crosses the surface, the effect is called refraction.
Normally, the brain estimates an object’s position by treating incoming light as if it traveled in a straight line. Light from the underwater part has already changed direction at the surface, so it seems to come from a different position. The two parts of the straw then appear not to line up.
Key point
The same refraction can make an underwater object look closer to the surface than it really is.