Ibn Sahl
Persian mathematician and optician who discovered the law of refraction (Snell's law) 600 years before Snell.
Who or what
Abu Sa'd al-Ala' ibn Sahl (c. 940–1000) was a Persian mathematician and optician who worked at the Abbasid court in Baghdad.
In his "Treatise on Burning Instruments," he described the law of refraction now known as Snell's law: .
Why it mattered
Ibn Sahl's discovery of the correct law of refraction was virtually unknown until the 20th century, when his manuscript was rediscovered in Tehran.
He used his law to design "anaclastic" lenses that focus light without spherical aberration.
Key works and moments
He described the relationship between the angle of incidence and refraction for light passing from air into glass and water.
His design of perfect focusing lenses (anaclastic lenses) required understanding the exact refraction law.
Further detail
His derivation used the concept of the ratio of distances along the interface, equivalent to the ratio of sines of the angles.
He expressed the law as: the ratio of the sine of incidence to the sine of refraction is constant for a given pair of media.
Legacy
Ibn Sahl's work underlies all modern lens design, fiber optics, and optical engineering.
His discovery predates Descartes and Snell by six centuries, yet was lost to history and rediscovered only in the 1990s.
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