Light slows down when it enters water and bends as it does, which is why a straw in a glass looks snapped in half.
Most people think a straw looks bent because the water magnifies or warps it. In fact the straw is perfectly straight; light slows entering the water and bends, so the underwater half no longer lines up with the dry half.
What's actually happening
Light has different speeds in different stuff: fastest in vacuum, a touch slower in air, about 25% slower in water, 33% slower in glass. When a beam crosses from one medium into another at an angle, one side of the beam slows down before the other, and like a shopping trolley with one wheel in the grass, the whole beam pivots.
That pivot is refraction, and it follows a precise rule (Snell's law): the deeper the speed change and the steeper the entry angle, the bigger the bend. Going into water, light bends toward the vertical; coming out, it bends away. Every ray leaving a submerged object is kinked at the surface on its way to your eye, but your brain, as always, traces straight lines. It reconstructs the object along the straightened path: shallower and slightly offset from where it really is.
This is why pools are always deeper than they look, only about three-quarters of their true depth, and why a fish seen from the bank is not where it appears. Herons and spear-fishers both learned to strike below the image. It is also the entire working principle of every lens on Earth: a lens is just refraction with the surface curved on purpose, so that all the bending converges to a point.
Light changes speed at the water line and bends as it does, so submerged things look shallower and shifted — which is why a pool is deeper than it appears.
- 1Drop a coin into an empty mug and step back until the rim just hides the coin from view.
- 2Hold your head perfectly still while someone slowly pours water into the mug.
- 3The coin rises into view. The water bends the rays from the coin down over the rim and into your eye: you are seeing around a corner via refraction.
Common questions
Light from the bottom bends as it leaves the water, but your brain traces straight lines and reconstructs the bottom along the straightened path. A pool therefore looks only about three-quarters of its true depth.
Refraction shifts a submerged fish to look shallower and slightly offset from where it actually is. To hit it, you must strike below the image rather than at it.
Hot air above tarmac is less dense and lets light travel faster, so rays bend gradually upward. The sky reflected off that bend looks like a shimmering puddle on the road.