Translation and discussion of Leon Brillouin's 1922 paper titled: Diffusion of light and X‐rays by a transparent homogeneous body: Influence of thermal agitation

Abstract In 1922, Léon Brillouin introduced the idea of light scattering from thermal fluctuations, in a paper titled: ‘Diffusion de la lumière et des rayons X par un corps transparent et homogéne, influence de l'agitation thermique’. Brillouin's mathematical treatment elegantly unifies principles from thermodynamics, optics and electromagnetic theory (as they were understood at the time) to arrive at a set of equations describing the scattering process, in a form that is still in use today. The effect he describes became known as Brillouin scattering, and was experimentally verified in 1930. It would go on to become the underlying principle of novel sensing and lasing technologies, as well as more recently also Brillouin microscopy. Despite the elegant derivation presented by Brillouin, and its scientific significance, the original French manuscript is rarely studied by the scientific community. With English serving as the lingua franca of our time, doing so is understandably challenging for the broader research community. While the original manuscript presents coherent and well‐founded theoretical derivations, following the thought process of the author, from a direct literal translation of the article can be somewhat challenging. The paper uses dated notations, some less common mathematical transformations, and in some cases seemingly unjustified physical simplifications. It also on occasion changes symbols for a quantity half way through, and there are several typographical errors that might confuse readers not intimately familiar with his line of argument. Here we provide an English translation of the original manuscript, with modernized notations and corrections, together with a supplementary text in which we discuss the derivation of each of the equations, along with the assumptions used in obtaining them. Our hope is that by providing the literal translation together with the accompanying discussion, we can transform this document from a blindly cited historical artefact to an accessible pedagogical tool that provides insight into the scientific development and discussions in the 1920's.