The link maintained, throughout the history of human thought in its diversity, between light and understanding, still in use in the language, has been actually transformed, with the elaboration of scientific knowledge, in a precise meaning of each of these terms, now distinct. On the one hand, light, conceived as material, belonging to the natural world, and object of study of the science of nature specified gradually as Physics, in which Mathematics play a growing role (but without confusing their respective objects), and of which Optics is a part. On the other hand, knowledge and understanding in the proper forms of human thought, organising themselves according to the function of reason, which makes possible to ‘transcend’ the experience of the world, in different directions, including that one which makes this world intelligible to us, and that one also, thought to be correlative of the first, which frees the mind (which is given by the maintained metaphorical expression ‘lights of reason’). It is the aim of the present essay to shed some light on two significant moments of the history of the relations of ‘physical’ light and rationality.

The first moment is that of the work in Optics by Ibn al-Haytham (end of the tenth and beginning of the eleventh centuries), which somehow inaugurates this history of physical light and the science of it, Optics: his work was to bear fruits various centuries later, contributing to inspire the Scientific Revolution of the seventeenth century. One finds in it precisely this foundational distinction between light and knowledge through the breaking with the traditional doctrine of the ‘visual ray’, the reasoned statement of the material nature of light, and the elaboration of a Physical and Geometrical Optics conceptually justified.

The second moment is that, nine centuries later, of the works of Albert Einstein concerning light, which are at a turning point, in the twentieth century, of the full consideration of the physical nature of light, a turn between two quite different conceptions of the constitution of material light and the corresponding theories, respectively, the ‘classical’ and the ‘quantum’ ones. The first one is the crowning of the classical Electromagnetic Theory of Light with the Theory of Special Relativity, obtained from a further explicitation of the physical nature of space and time, which hold and ‘carry’ light, and of the speed of the latter that becomes an invariant. The second one is the radical questioning of that ‘classical’ conception and theory with the discovery of new physical properties of light and of atomic matter. Einstein was the first to show that such a questioning was unavoidable, which determined the direction of his pioneer researches in this field, on the quantum nature of light related with that of atomic matter.

We shall conclude these evocations by some comparative or general remarks, on the relationship between Physics and rationality at different periods of the history of science, on the requirement of objectivity and the conditions of mathematisation, among other lessons that can be drawn from these particularly significant works on the physical nature of light and on the movements of its knowledge by rational thought.