Ibn Sahl

Ibn Sahl was a 10th-century mathematician and physicist who formulated a quantitative law of refraction and applied it to burning mirrors and lenses.

Law of refraction: A rule describing how light changes direction when it crosses between transparent media. Ibn Sahl expressed refraction through a geometric relation between incidence and transmission angles.
Refraction: The change in direction of a wave as it passes between media with different propagation speeds. This is the optical phenomenon Ibn Sahl analyzed quantitatively.
Book of Burning Instruments: Ibn Sahl's treatise on mirrors and lenses that concentrate sunlight. This work contains the refraction construction for which he is best known.
Burning mirror: A reflective surface shaped to concentrate sunlight at a focus. Ibn Sahl analyzed mirror forms intended to gather sunlight into intense heat.
Snell's law: The modern relation between the sines of incidence and refraction angles, scaled by refractive indices. It states refraction in a form that resembles Ibn Sahl's earlier geometric rule.
Reflection: The return of a wave from a boundary into the medium from which it arrived. Burning mirrors rely on reflection, another ray behavior central to his optical designs.
Arabic optics: The study of light developed by scholars writing in Arabic, especially during the medieval period. Ibn Sahl's treatise is an early landmark within this scientific tradition.
Lens: A transparent optical element that refracts light to form or alter an image. His calculations addressed lens shapes that could focus parallel rays.
Refractive index: A dimensionless measure of how strongly a medium slows light compared with vacuum. It quantifies the optical property underlying the bending described in Ibn Sahl's construction.
Conic section: A curve formed by intersecting a plane with a cone, including ellipses, parabolas, and hyperbolas. Conic geometry supplied shapes for mirrors and lenses that focus light.

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History of opticsRelated: His formulation anticipated the relation later known as Snell's law.