A straw partly submerged in water can look bent or broken because light from the underwater part changes direction as it passes from water into air. Your eyes assume the light traveled in a straight line, so they perceive the submerged section at a shifted position.
Light changes direction at the surface
Light travels at different speeds in water and air. When rays cross the boundary at an angle, they bend; this change in direction is called refraction. Rays from the underwater part of the straw bend as they leave the water. [1]
Your brain traces the rays backward
Your eyes and brain interpret incoming light as if it traveled straight. The refracted rays therefore seem to come from a point displaced from the straw’s true position. The part above the water is seen in its usual place, so the two sections no longer line up.
Why the straw looks normal when lifted out
Without the air-water boundary, light from the straw no longer bends at the surface. The apparent shift disappears. The same principle helps explain how lenses focus light and why a pool can look shallower than it is.
Does the straw actually bend?
No. The straw stays straight; refraction changes the apparent position of the portion under the water.
Sources and evidence
- The Physics Classroom — Refraction and Sight ↗
Demonstrates that a partly submerged straw appears misaligned because light refracts at the water surface.
- OpenStax College Physics — Snell’s Law ↗
Explains how light changes direction when it crosses between materials.
Sources consulted: 15 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; the explanation was checked against linked institutional, medical, or scientific references. This article was prepared with AI assistance and has not received independent expert review.