Alhazen was most original in his theory of light and vision, where he proceeded by experiment and induction, and again voiced skepticism of past authorities such as Euclid (300 BC) in his extramission theory of vision. Alhazen rejected the classical concept of the visual cone emanating from the eye, pointing to the error of assuming that some form of radiance could flow from the eye the moment it was opened and illuminate the entire heavens.
Alhazen earned his living by transcribing all the geometric works by Greek mathematician Euclid and selling it for cash. Incidentally he developed a technical mastery of straight lines and motion. He found employment at the court of the powerful caliph Al-Hakim in Cairo, who encouraged the enterprise of learning and technique in order as he obsessively wished to control all elements in the world around him. Desiring to command the Nile river, the caliph commissioned Alhazen to stop its annual flooding. Alhazen, though assuming the project, quickly determined its doom to failure and feigned insanity in order to escape the caliph's wrath. Unfortunately the plan did not succeed and he was sentenced to ten years in prison.
Under police control in miserly aphotic conditions Alhazen began to meditate on what he could see, becoming obsessed with light and dark. He questioned the classical idea, the accepted dogma for a thousand years, that sight was due to light coming out of a human eye and touching an object to produce vision (extramission). He countered that extramission cannot explain vision because a person experiences severe pain in the eye when looking at a very bright object like the sun, when such pain should not exist if the eye was the source, not the recipient, of light. Though extramission could be refuted, very little evidence existed supporting intromission as it was very difficult to argue that a coherent, miniature version of an entire object could be received by the human eye, the nature of the shortening, and location of the projection.
Work in optics
Alhazen surmized that if light was independent of the eye, some mechanism had to redirect light into the eye. He realized there was a clue in the function of mirrors, claiming that light rays must be brought back (or make the object send back) some kind of visual packet to the eye. Studying those patterns, Alhazen uncovered the symmetry that the angle of incident and reflected light are equal (the angle at which light hits an object is the same as its reflection), analogous to the way a ball struck at, and rebounded from, a wall.
He made a breakthrough in realizing that light reflects off of all objects not just mirrors and precisely developed via mathematics how light bounces off (reflection) and bends through (refraction) and object. He validated the intromission hypothesis of vision by explaining light from luminous bodies is received by all other objects, which also become luminous. Alhazen was the first to employ the concept of rays of light travelling in straight lines from every point on its source in all directions, a critical advance on the classical theory that objects emitted coherent, tiny copies of themselves.
In the year 1021, twelve years into his prison sentence, the caliph died and Alhazen was set free and produced seven volumes covering light and vision, transforming its mathematical rules and enacting the modern study of optics. His texts remained fundamental for centuries from the thirteen to the seventeenth centuries in the east and west, where Latin editions of Alhazen were studied by Kepler, Fermat and Descartes. Alhazen's book, De Aspectibus, assimilated the physical, mathematical, and physiological aspects of vision.
Unfortunately, Alhazen considered colors distinct from white light, did not provided a satisfactory justification how light bent in the eye (refracted light) is perceived.