Jim Al-Khalili on Ibn al-Haytham's "Book of Optics"

by Salman Hameed

I didn't know this is the International Year of Light (IYL 2015). In one way we can celebrate IYL by celebrating astronomy. With the exception of landers and rovers, all of our information comes through light - and even with rovers, the communication is through light. Nevertheless, as part of IYL (I think), Nature published this pieced by Jim Khalili on al-Haytham's Book of Optics (unfortunately, Jim starts the article by declaring him to be a physicist - which is anachronistic):

The greatest physicist of the medieval era led a life as remarkable as his discoveries were prodigious, spending a decade in prison and at one point possibly feigning mental illness to get out of a tight spot. Abu Ali al-Hassan ibn al-Haytham (Latinized to Alhazen) was born in Basra, now in southern Iraq, in AD 965. His greatest and most famous work, the seven-volume Book of Optics (Kitab al-Manathir) hugely influenced thinking across disciplines from the theory of visual perception to the nature of perspective in medieval art, in both the East and the West, for more than 600 years. Many later European scholars and fellow polymaths, from Robert Grosseteste and Leonardo da Vinci to Galileo Galilei, René Descartes, Johannes Kepler and Isaac Newton, were in his debt. Indeed, the influence of Ibn al-Haytham's Optics ranks alongside that of Newton's work of the same title, published 700 years later.

And of course, whether true or not, you have to recount the story of his "mental illness":

As a young man, Ibn al-Haytham received an excellent education and was widely noted as a mathematical and scientific prodigy. Frustrated by his administrative duties working in a government post in the vast Islamic Empire — which at the time stretched from India to Spain — he was sacked owing to real or, as some speculate, faked mental illness. 

Sometime during the first decade of the new millennium, he proposed an ambitious project to dam the Nile. He was invited to Egypt by the Fatimid caliph al-Hakim bi'amr Illah. However, on seeing the scale of the task, Ibn al-Haytham quickly realized that it was beyond him. He was promptly imprisoned in Cairo for wasting the caliph's time. 

Far from cowing him, the decade of imprisonment granted Ibn al-Haytham the seclusion to think and write, particularly on optics. After his release around the year 1020, he began working at a prolific rate, carrying out a series of famous experiments on the nature of light. For example, using a camera obscura, he proved that light travels in straight lines; he also mathematized the fields of catoptrics (reflection of light by mirrors) and dioptrics (refraction of light through lenses). This huge body of experiment and theory culminated in his Book of Optics.

Here are the specifics of his Book of Optics:

This treatise can be regarded as a science textbook. In it, Ibn al-Haytham gives detailed descriptions of his experiments, such as exploring how light rays are reflected off plain and curved surfaces. He includes the apparatus he used, the way he set it up, the measurements and his results. He then uses these observations to justify his theories, which he develops with geometrical models. He even urges others to repeat his experiments to verify his conclusions. Many historians of science consider Ibn al-Haytham to be the first true proponent of the modern scientific method. 

The work can be roughly divided into Books I, II and III, devoted to the theory of vision and the associated physiology of the eye and the psychology of perception; and Books IV to VII, covering traditional physical optics. The work's most celebrated contribution to science is its explanation of vision. 

At that time, scholars' understanding of the phenomenon was a mess. The Greeks had several theories. In the fifth century BC, Empedocles had argued that a special light shone out of the eye until it hit an object, thereby making it visible. This became known as the emission theory of vision. It was 'refined' by Plato, who explained that you also need external light to see. Plato's student Aristotle suggested that rather than the eye emitting light, objects would 'perturb' the air between them and the eye, triggering sight. 

Other philosophers around this time, including Epicurus, attempted a form of 'intromission theory' of vision (light entering the eye from outside), but it was Plato's theory that was given a mathematical basis by Euclid, who described light rays emerging in a cone from the eye. Several centuries later, Ptolemy expanded on this idea. 

Early Islamic scholars such as al-Kindi and Hunayn ibn Ishaq favoured a combined emission–intromission theory. They posited that the eye sends out light to the observed object, which then reflects the light back into the eye. 

It took the genius of Ibn al-Haytham to finally resolve the issue. He argued that if we see because rays of light are emitted from the eye onto an object (Plato and Euclid's 'sight rays'), then either the object sends back a signal to the eye or it does not. If it does not, how can the eye perceive what its rays have fallen on? Light must be coming back to the eye, and this is how we see. But if so, what use is there for the original rays emitted by the eye? The light could come directly from the object if it is luminous or, if it is not, could be reflected from the object after being emitted by another source. Rays from the eye, decided Ibn al-Haytham, are an unnecessary complication. 

He also went further than anyone before in trying to understand the underlying physics of refraction. He argued that the speed of light was finite and varied in different media, and he used the idea of resolving the path of a light ray into its vertical and horizontal components of velocities. He carried out all his work geometrically, and introduced many new ideas, such as the study of how the atmosphere refracts light from celestial bodies.

Read the full article here (you may need subscription to access it). You can also read more about al-Haytham on IYL 2015 website.