Why does a straw in a glass of water look bent or broken at the surface?
Light from the underwater part of the straw bends as it leaves the water (denser) into air (rarer), refracting away from the normal.
Light from the underwater part of the straw bends as it leaves the water (denser) into air (rarer), refracting away from the normal. Your eye traces those rays back in straight lines, so the submerged part appears raised and shifted — the straw looks bent at the waterline. Nothing physically changed; only the light path did.
Key point
Refraction at the water–air surface shifts the apparent position of the submerged part.
Common mistake
thinking the straw or the water actually changes shape.
Memory tip
your eye assumes straight-line light, so refraction fakes a bend.
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