Diagram illustrating sine law of refraction. From La Dioptrique by Rene Descartes, 1637. Descartes (1596-1650) was a French mathematician and philosopher. He is considered a founding father of modern philosophy, famous for his statement "Cogito ergo sum" - "I think, therefore I am" (from his 'Discourses on the Method' 1637). Snell's law (also known as the Snell-Descartes law and the law of refraction) 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. Although named after Dutch astronomer Willebrord Snellius (1580-1626), the law was first accurately described by the scientist Ibn Sahl at the Baghdad court in 984. Descartes independently derived the law using heuristic momentum conservation arguments in terms of sines in his 1637 essay Dioptrics, and used it to solve a range of optical problems.