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Light & Colour · Question 06 of 20

Refraction in water

Light changes speed at the water line — and that single fact bends straws, sinks coins, and steals fish from spear-fishers.

Plate IX — Snell's law, live n₁ sin θ₁ = n₂ sin θ₂
Steepen the beam, then try diamond — the harder light brakes, the harder it swerves.
water · light at 75% speedair · full speednormal48°34.0°slower medium → the ray pivots toward the normal
FIG. IX — SNELL'S LAW, LIVE
Incoming angle 48° from the normal
The slower stuff n = 1.33
Bent to
34.0°
Light's speed inside
75% of air
Light slows down in water, and slowing at an angle makes it swerve — like a trolley with one wheel in the grass. That kink is why a straw looks snapped and why a pool is always deeper than it looks. Try diamond: the hardest swerve of all.
The short answer

Light slows down when it enters water and bends as it does — which is why a straw in a glass looks snapped in 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 — a pool looks about three-quarters of its 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.

Try it at home The reappearing coin
  1. 1Drop a coin into an empty mug and step back until the rim just hides the coin from view.
  2. 2Hold your head perfectly still while someone slowly pours water into the mug.
  3. 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.