Ibn Sahl

"This article is about the physicist. For the physician, see Ali ibn Sahl Rabban al-Tabari. For the poet, see Ibn Sahl of Sevilla."

Ibn Sahl (Abu Sa`d al-`Ala' ibn Sahl) (c. 940-1000) was an Arabian mathematician, physicist and optics engineer of the Islamic Golden Age associated with the Abbasid court of Baghdad. About 984 he wrote a treatise "On Burning Mirrors and Lenses" in which he set out his understanding of how curved mirrors and lenses bend and focus light. Ibn Sahl is credited with first discovering the law of refraction, usually called Snell's law. [K. B. Wolf, "Geometry and dynamics in refracting systems", "European Journal of Physics" 16, p. 14-20, 1995.] R. Rashed, "A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses", "Isis" 81, p. 464–491, 1990.] He used the law of refraction to work out the shapes of lenses that focus light with no geometric aberrations, known as anaclastic lenses.

In the reproduction of the figure from Ibn Sahl's manuscript, the critical part is the right-angled triangle. The inner hypotenuse shows the path of an incident ray and the outer hypotenuse shows an extension of the path of the refracted ray if the incident ray met a crystal whose face is vertical at the point where the two hypotenuses intersect. [Kurt Bernardo Wolf, "Geometric Optics on Phase Space", p. 9, Springer, 2004, ISBN 3540220399 [http://books.google.com/books?vid=ISBN3540220399&id=KmpCDg-D39gC&pg=PA8&lpg=PA8&ots=NJM71aAIlg&dq=%22ibn+sahl%22&sig=USHa-lMN4NCsoQrr5qNM0_4fr1A#PPA9,M1 online] ] According to Rashed, the ratio of the length of the smaller hypotenuse to the larger is the reciprocal of the refractive index of the crystal.

The lower part of the figure shows a representation of a plano-convex lens (at the right) and its principal axis (the intersecting horizontal line). The curvature of the convex part of the lens brings all rays parallel to the horizontal axis (and approaching the lens from the right) to a focal point on the axis at the left.

In the remaining parts of the treatise, Ibn Sahl dealt with parabolic mirrors, ellipsoidal mirrors, biconvex lenses, and techniques for drawing hyperbolic arcs.

Ibn Sahl's treatise was used by Ibn al-Haitham (965–1039), one of the greatest Arabic scholars of optics. In modern times, Rashed found the manuscript to have been dispersed over two libraries. He reassembled it, translated it, and published it. [Rashed, R., "Géométrie et dioptrique au Xe siècle: Ibn Sahl, al-Quhi et Ibn al-Haytham." Paris: Les Belles Lettres, 1993.]

ee also

*History of optics
*Islamic science
*List of Arab scientists and scholars

References

External links

* [http://unesdoc.unesco.org/images/0014/001412/141236E.pdf The Miracle of Light - a UNESCO article on the history of optics]