Léon Brillouin (7.08.1889 — 1969) was a French physicist. He was born in Sèvres. He graduated from the École Normale (1912). He taught radiophysics at the Higher School of Electrical Engineering (1921—31) and theoretical physics at the Henri Poincaré Institute (1928-32), and from 1932 he was head of the chair of theoretical physics at the Collège de France. In 1939 —40 he was director of French radio broadcasting. From 1941 he worked at universities in the USA, in particular from 1954 at Columbia.
His main research was in the fields of quantum mechanics, radiophysics, solid-state physics, cybernetics and the philosophical problems of science. In quantum mechanics he is known for the Brillouin-Wigner formula, for Brillouin's theorem, which is connected with the Hartree-Fock self-consistent field method, and for the WKB method (the Wentzel-Kramers-Brillouin method), which is widely used, particularly in quantum chemistry. In 1922 he predicted the change in the fine structure of a spectrum in the fluctuation scattering of light, known as the Brillouin-Mandelstam effect (in 1926 L. I. Mandelstam published similar results). In 1927 he explained Langevin's theory of paramagnetism from the quantum standpoint, introducing the so-called Brillouin function. He introduced into the physics of crystals the important concept of Brillouin zones. Together with F. Bloch he laid the foundations of the band theory of solids. Independently of P. Debye he predicted (1914) the diffuse scattering of X-rays by lattice vibrations. He also worked on information theory and the philosophical questions of science.
A member of the National Academy of Sciences of the USA (1953).