Willebrord Snell

Willebrord Snell (1580–1626) was a Dutch astronomer and mathematician who pinned down the exact rule for how light bends when it passes from one material into another. It's the reason a straw looks broken in a glass of water and a swimming pool always looks shallower than it really is — a little everyday magic trick that Snell reduced to one tidy equation.

The idea that stuck

Snell's law is the precise recipe for reflection and refraction: it says exactly how much a ray of light kinks when it crosses from, say, air into glass or water. Push the angle far enough and the light stops crossing over altogether and bounces entirely back inside — total internal reflection, the effect that makes diamonds sparkle and lets fibre-optic cables pipe the whole internet around the planet as trapped flashes of light. Lenses, cameras, glasses and microscopes all obey his rule.

Snell's law has one of the messiest ownership histories in physics. Snell worked it out around 1621 but never published it — René Descartes printed the same rule years later, so in French it's often called "Descartes' law." Dig further back and a Persian scholar, Ibn Sahl, had essentially nailed the same relationship over six centuries earlier, in the year 984. So the neat little equation on your physics worksheet is really the work of many minds across a thousand years, each rediscovering the same beautiful bend in a ray of light.