Before his relocation to Cairo, Ibn al-Haytham likely began formulating his ideas about vision and optics. This period involved studying existing theories, including those of Euclid, Ptolemy, and Galen, and identifying their inconsistencies and limitations. He began to formulate his new theory of vision based on the emission of rays "from" an object "to" the eye, rather than the reverse.

The majority of Ibn al-Haytham's most important work on optics was conducted during his time in Cairo, primarily during his period of restricted living under al-Hakim. It was during this time he wrote the seven volumes of "Kitab al-Manazir". It presents a comprehensive and systematic investigation of light, vision, and related phenomena, based on rigorous experiments and mathematical analysis. Key concepts developed in this book include: his intromission theory of vision (the eye sees because rays come from observed objects and not vice versa); refraction of light through transparent media, especially lenses; the first correct explanation of visual perception and depth; a theory of color mixing; and the earliest known accurate description and analysis of the camera obscura.

Ibn al-Haytham's meticulous experimentation with the camera obscura (a dark room with a small hole that projects an inverted image of the outside world) provided crucial evidence for his theory that light travels in straight lines. His detailed observations of the camera obscura's properties and image formation profoundly impacted the understanding of optics.

Ibn al-Haytham dedicated significant effort to studying the refraction of light as it passes through different media, such as water and glass. His experiments led to improved understanding of the bending of light rays and the formation of images through lenses. He also thoroughly investigated reflection from mirrors, both flat and curved, contributing to the development of catoptrics (the study of mirrors).