Physics
Wave Refraction
Quick fact
Refraction causes the apparent depth of a swimming pool to be shallower than its actual depth; light bends when leaving water, tricking your eyes.
Why this is interesting
Have you ever noticed how a straight stick looks bent when you put it in a glass of water? That's the same effect that makes waves change direction as they move from deep to shallow water.
Read the full explanation
Understanding Wave Refraction
Imagine a line of marching soldiers holding hands. If they march from pavement onto mud at an angle, the soldiers on the mud slow down first, causing the whole line to turn. That's refraction: a wave bends because part of it travels slower (or faster) than the rest. The wave changes direction at the boundary between two materials, like air and water, or deep and shallow water. The key idea is that the wave's frequency stays the same, but its speed and wavelength change. The bending is always towards the slower medium if the wave enters at an angle.
A deeper explanation
Refraction is governed by Snell's law: n₁ sin θ₁ = n₂ sin θ₂, where n is the refractive index (a measure of how much a medium slows light relative to a vacuum). The underlying mechanism is described by Huygens' principle: every point on a wavefront acts as a source of small secondary waves. When part of the wavefront enters a new medium at a different speed, these secondary waves propagate at different rates, causing the wavefront to pivot. This matters because refraction enables lenses to focus light, makes prisms disperse colours, and allows fibre optics to guide light. In oceanography, it explains why waves turn to hit coastlines more head-on. Without refraction, glasses, cameras, and even rainbows would not exist.