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 replication of Hippolyte Fizeau's method to measure the speed of light (see Fizeau's measurement of the speed of light in air), introducing various improvements in the apparatus, which added greatly to the accuracy of the results.

Cornu was the President of the Société Astronomique de France (SAF), the French astronomical society, from 1897-1899.

[3] 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.

[4] The Cornu spiral, a graphical device for the computation of light intensities in Fresnel's model of near-field diffraction, is named after him.