Ibn Sahl
Willebrord Snellius (1580–1626) found the mathematical law of refraction, now known as Snell's law, in 1621. It had been published earlier in 984 A. D. by Ibn Sahl. Subsequently René Descartes (1596–1650) showed, by using geometric construction and the law of refraction (also known as Descartes' law), that the angular radius of a rainbow is 42° (i.e. the angle subtended at the eye by the edge of the rainbow and the rainbow's centre is 42°). He also independently discovered the law of reflection, and his essay on optics was the first published mention of this law.