Ibn Sahl (Arabic: ابن سهل) (Abu Saʿd al-ʿAlaʾ ibn Sahl ) (Arabic: أبو سعد العلاء ابن سهل) (c. 940–1000) was a Muslim mathematician, physicist and optics engineer of the Islamic Golden Age associated with the Abbasid court of Baghdad. Ibn Sahl's 984[1] treatise On Burning Mirrors and Lenses sets 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.[2][3] He used the law of refraction to derive lens shapes that focus light with no geometric aberrations, known as anaclastic lenses.