Put the lamp back
In early eleventh-century Cairo, a lamp was moved in front of a mirror. Ibn al-Haytham marked the point where its light reached the surface, drew a normal there, and measured the lines on both sides. Then he moved the lamp, drew the construction again, and repeated the test.
It is not the usual shape of a heroic tale. It is a method that is hard to undo. Euclid and Ptolemy could tell him what ought to happen. The mirror let him check their words. Light arrived from one side and left on the other; both angles had to survive the same ruler.
Diagram description
Static physics diagram: Lamp and mirror.
The diagram reconstructs the tabletop experiment in the story. The lamp, mirror, and normal are all present. Change incidence_angle and the reflected ray is redrawn. A plane-mirror figure is no longer standing in for an unseen dark-room apparatus.
The eye stops reaching outward
For centuries, theories of vision often began by asking what the eye sent outward to touch the world. Ibn al-Haytham reversed the direction. Light traveled from luminous or illuminated objects toward the eye. Mirror experiments tested reflection; a dark room with a small aperture tested straight-line propagation. Different arrangements closed in on the same account.
His Book of Optics, completed around 1021, joined geometrical proofs, controlled arrangements, and questions about perception. Respect for Euclid and Ptolemy did not prevent him from doubting them. It made him read closely enough to know where to move the lamp so that an old explanation had to face a repeatable observation.
Historical source: UNESCO: Ibn al-Haytham’s scientific method.