A simple ratio predicts exactly how much light bends entering water

When light passes from one transparent material into another, such as air into water, it bends at the boundary. Dutch astronomer Willebrord Snellius worked out in 1621 that the ratio of the sines of the entry and exit angles is always a fixed constant for any given pair of materials, a relationship now called Snell's Law. The same mathematics, described even earlier by the mathematician Ibn Sahl around 984, underlies how every lens, prism, and camera bends light to a precise, predictable point.

— Willebrord Snellius, Dioptrice — Manuscript, 1621, published posthumously