Marie Alfred Cornu (March 6, 1841—April 12, 1902) was a French physicist.
Cornu was born at Orléans, and after being educated at the École polytechnique and the École des mines, in 1867 he became professor of experimental physics in the former institution, where he remained throughout his life. Although he made various excursions into other branches of physical science, undertaking, for example, with Jean-Baptistin Baille about 1870 a repetition of Cavendish's experiment for determining the gravitational constant G, his original work was mainly concerned with optics and spectroscopy. In particular he carried out a classical redetermination of the speed of light by A. H. L. Fizeau's method (see Fizeau-Foucault Apparatus), introducing various improvements in the apparatus, which added greatly to the accuracy of the results. This achievement won for him, in 1878, the prix Lacaze and membership of the Academy of Sciences in France, and the Rumford Medal of the Royal Society in England. In 1892, he was elected a member of the Royal Swedish Academy of Sciences. In 1899, at the jubilee commemoration of Sir George Stokes, he was Rede lecturer at Cambridge, his subject being the wave theory of light and its influence on modern physics; and on that occasion the honorary degree of D.Sc. was conferred on him by the university. He died at Romorantin on April 12, 1902.
The Cornu spiral, a graphical device for the computation of light intensities in Fresnel's model of near-field diffraction, is named after him. The spiral (or clothoid) is also used in geometrical road design. The Cornu depolarizer is also named after him.