Willebrord Snellius was a Dutch astronomer and mathematician, commonly known as Snell. His name is usually associated with the law of refraction of light known as Snell's law.
Willebrord Snel van Royen (1580–1626)
Quadrant of Snellius Image: Museum Boerhaave, Leiden
Snellius' Triangulation (1615)
Commemorative plaque on Snellius' house in Leiden
Snell's law is 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.
In optics, the law is used in ray tracing to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a material. The law is also satisfied in meta-materials, which allow light to be bent "backward" at a negative angle of refraction with a negative refractive index.
Reproduction of a page of Ibn Sahl's manuscript showing his discovery of the law of refraction
Snell's law on a wall in Leiden