Snell's Law

Discovered by: First accurately described by Ibn Sahl in 984 the law was named after Willebrord Snellius who derived an equivalent form in 1621.

What is it? A formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.

Why is it important? Used in optics to predict how lenses will affect light rays.