Why a straw looks bent in water

A straight straw can look bent at the waterline even while the physical straw keeps a straight shape. 1

The reason is refraction: light changes direction when it passes from one transparent material into another. 2 Light travels more slowly in water than in air, so a ray crossing the air-water boundary bends. 1

Your eyes receive that bent ray, while your brain estimates the straw's position by extending the incoming ray backward in a straight line. 3 That estimate places the underwater part higher and to the side, creating the familiar bent-straw look. 3