He built optics on experiment, in a Cairo dark room
Ibn al-Haytham (c. 965–1040), writing his Book of Optics in Cairo around 1011–1021, argued that vision results from light entering the eye and gave one of the first clear descriptions of the camera obscura, the darkened chamber that projects an inverted image through a small hole. Translated into Latin as De aspectibus, his work later informed European students of light including Johannes Kepler (1571–1630).
How we know
Ibn al-Haytham b. c.965 Basra, d. c.1040/1041 Cairo; Kitāb al-Manāẓir (Book of Optics) written in Cairo c.1011–1021; intromission theory + dark-room (al-bayt al-muẓlim) camera-obscura description; Latin De aspectibus (late 12th–early 13th c.) influenced Bacon, Witelo, Kepler (1571–1630, optical work c.1604).
One of many fact-checked wonders across eight traditions at The Spring of Wisdom.