February 13, 2025, 11am, CEMES (coffee offered at 10:30 am)

Conference room + Zoom 

by Florent Houdellier, CEMES-CNRS, INSA Toulouse

A thousand years ago, the world witnessed a major revolution in optics. The study of light by the Arab scholar Ibn al Haytham marked the birth of a revolutionary experimental approach with observation and analysis protocols, and the use of mathematics to formalise them. This revolution led to the awakening of the sciences in Europe throughout the Middle Ages, from the Scholastic period through to the Renaissance [1].

Hellenistic thought in antiquity was more interested in the phenomenon of vision than in the nature of light. Their theories can be roughly divided into three categories. The extramission theories attributed to Euclid and the mathematical school of Alexandria, in particular Ptolemy, required visual rays to be emitted by the eye. Euclid established the laws of reflection, and his book Optics and Catoptrics is centred on this concept of visual rays [2]. The theories of intromission, whose most eminent advocate was Aristotle, defend the presence of a potentially transparent medium, called the diaphanous medium, which becomes transparent through the action of light [3]. Finally, mixed approaches were also defended by Plato and Galen, and to some extent by Aristotle too [4]. All these theories, which prevailed until the end of the first millennium AD, shared the fundamental principle that physical contact between the eye and the object is necessary for vision.

The fall of Rome in 476 and the advent of Islam were major social and political events that profoundly transformed knowledge and thought [5]. The rapid expansion of Islam, first under the Umayyad dynasty and then under the Abbasid dynasty (750-1258), led to profound changes in the development of ideas. The Caliph Al-Ma’mūn (813-833) established one of the first centres of scientific research in history in Baghdad, the famous Bayt al-Hikma (House of Wisdom) [6]. Intense intellectual activity prevailed, with the translation, study and commentary of Greek, Persian and Indian texts. Hunayn Ibn Ishaq (808-873), a Nestorian Christian, trilingual (Arabic, Syriac and Greek) and an authority on the translation of Greek works, was one of the symbols of this period of translation. He also wrote the book ‘Ten Treatises on the Eye’ in the wake of Galen [7]. During this period, major scholars contributed to the development of optics as a science: Al Kindi (801-873), Ibn Sīnā (980-1037), Ibn Sahl (940-1000) and above all Abu Ali al-Hassan ibn al-Hassan Ibn al Haytham (965-1040) thanks to his major book ‘Kitāb fī al-manāẓir’ (Book on Optics) [8]. Ibn Sahl in his book ‘Kitāb al-ḥarrāqāt’ (Book on Burning Instruments) published in 984, for example, contributed the first mathematical description of the anaclastic properties of plano-convex and biconvex lenses (see Figure 1). This led him to deal with the refraction of light by a plane surface separating two media (see figure 2). He thus formulated the law of sines five centuries before Willebord Snell and René Descartes [9].

A few years ago, I came across the work of Dr. Roshdi Rashed, a researcher at the CNRS SPHERE laboratory, who was the first to study Ibn Sahl’s manuscript, as well as other revolutionary studies of geometrical optics by Ibn al Haytham [10].

I was simply blown away by what I discovered, both in terms of the depth of physical concepts in optics in the Islamic world at the time, and the modern methods used to study them. It was all the more shocking because the message ran counter to what I had more or less always heard in my scientific career, which was that Descartes and the Enlightenment were the gateway to the modern scientific method.

Since then, as an amateur, I have continued my investigations into these questions, which fascinate me. They also inform my approach to optics when, for example, I present its history on various occasions. I have included an introduction to the evolution of ideas about vision, light and optics in general in my courses at INSA.

I propose to share these astonishments with you during a Thursday presentation, and perhaps it will move you as much as it did me!

Figure 1 (left): The bi-convex lens
Figure 2 (right): and the plano-convex lens: extracts from Ibn Sahl’s treatise on burning instruments (Milli Library, Teheran) [9].

[1] M. Imbert La fin du regard éclairant. 2020. Vrin Mathesis.

[2] A. Lejeune. Euclide et Plolémée. Deux stades de l’optique géométrique grecque.1948. Bibliothèque de l’Université de Louvain,

[3] A. Vasiliu. Du diaphane : image, milieu, lumière dans la pensée antique et médiévale. 1997. Vrin – Études de philosophie médiévale

[4] A. Merker. La vision chez Platon et Aristote. 2003. Academia Verlag – International Plato Studies

[5] A. Djebbar. Une histoire de la science arabe. 2001. Points

[6] H. Touati, Bayt al-hikma : la Maison de la sagesse des Abbassides, in Houari Touati (éd.), Encyclopédie de l’humanisme méditerranéen, 2014 (http://www.encyclopedie-humanisme.com/?Bayt-al-hikma)

[7] Gül A.Russell, Histoire des sciences arabes, tome 2: Mathématiques et Physique. 1997. dirigé par R. Rashed. Éditions du Seuil.

[8] R. Rashed. Ibn al-Haytham L’émergence de la modernité classique. 2021. Hermann Philosophie, Politique et Économie – Sciences et Technique

[9] R. Rashed. Géométrie et dioptrique au xe siècle: Ibn Sahl, al-Qūhī, et Ibn al-Haytham. 1993. Collection Sciences et Philosophie Arabes, Textes et Études. Les Belles Lettres

[10] D. Gazagnadou. Entretien avec Roshdi Rashed. 2023. Edition Kimé