Snells

Snells is the plural form of the surname Snell and is used in reference to people or topics associated with that name. In science, the best known use is Snell's law, the law of refraction, which describes how light changes direction when crossing the boundary between two media with different refractive indices. The law is commonly written as n1 sin θ1 = n2 sin θ2, with θ measured from the normal to the interface. It was derived by Willebrord Snellius (also spelled Snellius or Snell), a Dutch mathematician and astronomer who lived circa 1580–1626, and published in 1621. Snell's law underpins the design of lenses, optical instruments, and fiber-optic systems.

A related phenomenon is Snell's window, observed underwater. Light from above the surface is refracted into the water and exits within a cone whose half-angle equals the critical angle for total internal reflection, creating a bright circular region visible from beneath.

The surname Snell or its plural form Snells appears across various cultures and is associated with multiple individuals who have contributed to science, medicine, literature, and other fields. In historical texts, the Latinized form Snellius is used for Willebrord Snellius, while modern references often use Snell or Snell(s). Thus, Snells as a term functions primarily as an eponymic marker linked to Snellius and to generic uses of the surname in encyclopedic contexts.