Ibn al-Haytham Rewrites the Science of Vision in Cairo
The Book of Optics proves that sight works by light entering the eye, not rays leaving it
Quick facts
- Scholar
- Ibn al-Haytham (Alhazen), c. 965-1040 CE
- Location
- Cairo, Egypt
- Key work
- Book of Optics (Kitab al-Manazir)
- Core finding
- Vision results from light entering the eye, not rays leaving it
What happened
Ibn al-Haytham, born around 965 CE and known in Latin as Alhazen, spent years working in Cairo, reportedly confined for a period under house arrest, where between roughly 1011 and 1021 CE he composed his seven-volume Book of Optics (Kitab al-Manazir). In it he broke with the older Greek theory that vision worked by rays emitted from the eye, instead demonstrating through geometric analysis and constructed instruments that light travels from an object to the eye and is refracted and reflected according to fixed laws. He built and used a copper instrument to measure how light reflects from flat, spherical, cylindrical, and conical mirrors, and he proposed that Earth's atmosphere had a finite depth of about 15 kilometers, using it to explain the timing of twilight.
Why it matters
The Book of Optics established experimental method, testing a hypothesis by constructed apparatus and measurement, as a way to settle a scientific question, work historians of science treat as a foundation of the modern experimental approach in physics. Its account of vision and reflection influenced both later Islamic scholars and, once translated into Latin, European scientists including Kepler.
How we know
Ibn al-Haytham's biography, his years in Cairo, and the content of the Book of Optics, including his instrument for measuring reflection and his atmospheric depth estimate, are documented in the MacTutor History of Mathematics archive maintained by the University of St Andrews.
Sources
- MacTutor History of Mathematics, University of St Andrews. Ibn al-Haytham · Reputable sourcemathshistory.st-andrews.ac.uk · The domain "mathshistory.st-andrews.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- MacTutor History of Mathematics, University of St Andrews. Ibn al-Haytham · Reputable sourcemathshistory.st-andrews.ac.uk · The domain "mathshistory.st-andrews.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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