About Alhacen and the Book of Optics

The Book of Optics is a medieval multiple-volume text pared by Alhacen, an 11th-century Arab physicist. (Alhacen is his Europeanized or Latin name; to the Muslim world, he was known by his given name, Ibn al Haytham.) Despite the name, the Book of Optics was as much about fundamental physics and properties of light as it was about vision and the human eye itself. For this reason the Book of Optics is one of the most important texts in the early history of science, although it is much less well known than European counterparts.

– An Overview of the Book of Optics –

The Book of Optics was completed by Alhacen over a decade-long period in the 1010s A.D., which he spent as a political prisoner in Egypt, having been jailed by the Fatimid government after his excessively ambitious attempt at damming and controlling the Nile River failed. The book was a veritable tour de force in the study of the physics of light, as well as being important for the series of principles for the study and correction of vision which followed.

Over seven full volumes, Alhacen developed a new theory of light, rejecting early classical theories that human sight was either the product of light beams produced by the eye (essentially a visual counterpart to bats’ and cetaceans’ echolocation) or that it was a physical substance absorbed by the eye. Instead, he argued, light was a ray which travelled through the natural world in straight lines. This generalization was the foundation for subsequent studies of light, and featured the documentation of a wide variety of important experiments involving mirrors and lenses.

Originally an Arabic text, the Book of Optics was found and then translated by Italian philosophers the following century, just prior to the opening of the Italian Renaissance (in general, a period which featured European philosophers picking up, duplicating, and eventually adding to theories originating in the Islamic world). It circulated quite widely in Renaissance intellectual circles.

Google Books carries full translations of parts of Alhacen’s work.

– Significance of the Book of Optics –

The Book of Optics is most significant for three contributions: its theoretical contributions to physics, its understanding of the human eye, and its advancement of the philosophically mechanistic and methodologically experiment-driven process which we, today, know as the so-called “scientific method.”

The previous intellectual leadership in philosophy and science, that of the ancient classical period, had been European and specifically Mediterranean. (Separate intellectual traditions developed in India and East Asia.) However, during the medieval period, war and strife dominated Europe and intellectual development instead moved to the comparative safe haven of the Muslim world, to the south. Alhacen’s work reflected this trend, challenging as it did the previously accepted thought of Ptolemy (that light was a ray emitted by the eye and then bounced back by objects, creating “vision” in much the same way that radar and sonar now operate), and Aristotle (that light was a physical substance emitted by objects). Instead, Alhacen argued, it was a ray of light which always travelled in a straight line from the point of emission.

If light was a naturally emitted ray, this held major implications for our understanding of the human organ which receives and interprets light: the eye. The eye was not a sort of organic radar device (comparable to echolocation in bats), nor was it the site of physical interaction between physical light and the human body. The correct interpretation of the function of the eye allowed for an unprecedentedly detailed (if still in many ways speculative) analysis of the different parts of the eye, in which Alhacen explored for the first time the physical makeup of the eye and the ways in which its components operated, as well as how they contributed to human vision (for example, through the importance of two-eyed or binocular vision, and through the realization that human sight relied not only on reception of light but by its processing or interpreting in the brain, transmitted via the optic nerve).

Finally, the Book of Optics was significant because of its basis in experimental observation and inferences drawn from those observations – in other words, the sort of process which is now recognized as the scientific method. Various experiments were described in the book as the basis for Alhacen’s conclusions, involving commonly understood devices such as mirrors and lenses but also equipment invented by Alhacen and subsequently commonly used by scientists, such as the camera obscura, a primitive light reception and projection device which formed the basis for the invention of projection and photography.

Experimentation was not new: ancient Greek philosophers made similar advances. However, the late Muslim and early Renaissance scholarship which emerged after the Middle Ages can be understood as a major and permanent revival of physical experiments rather than religious or philosophical theory as the basis for building an understanding of the natural world. The revival of experimentation reached its climax in the Enlightenment, with the rise of the academic discipline of science itself. Alhacen and the Book of Optics are not as well-recognized as contributors to this process as his later European counterparts, such as astronomers Galileo and Copernicus, but nevertheless played a significant role in building an appreciation of experimental science in the early modern period.