Does light leave the eye, or enter it?

That time, that place

Cairo. Ibn al-Haytham took on a project to control the Nile floods and realized it could not be done. Reporting failure would have cost him his life, so he feigned madness and lived confined until the caliph died. In that confinement, he studied light.

Why this question

Two accounts of seeing were in contention: that something reaches out from the eye to touch the object, or that something leaves the object and enters the eye.

For centuries there had been argument and no way to settle it. What al-Haytham did was not to join the argument but to build a way of settling it.

What was found

He began with the simple things. Stare at something bright and your eyes hurt. Look at the Sun and an afterimage lingers. If something goes out from the eye, why is the eye the thing that is injured?

The decisive case was the darkened room. Block every window but one small hole and the scene outside appears, inverted, on the opposite wall. A wall is not an eye. If an image forms on a wall that emits nothing, then light must come from the object and travel straight in.

In his Book of Optics he wrote that whoever examines a claim must doubt what he has read and verify it by experiment.