Why a Straw Looks Bent in Water

Put a straight straw in a glass of water and the waterline seems to snap it. The straw remains straight; the submerged part appears shifted because light from that section changes direction as it leaves water and enters air.

That change is refraction. Light speeds up in air and bends away from an imaginary line perpendicular to the surface, called the normal. Your eyes trace the final ray backward as though the ray had traveled straight, so the submerged section appears to come from a different position.

Water’s refractive index is about 4/3, and the same relationship makes a pool look about 3/4 as deep as its real depth when viewed from above.123

Photo: Pexels.