Gouy, Louis-Georges

(b, Vales-les-Bains, Ardèche, France, 19 February 1854; d, Vales-les-Bains, 27 January 1926)

general physics, optics.

Almost nothing is known of Gouy’s upbringing and education. He spent most of his productive life as a professor at the Faculty of Sciences at the University of Lyons. He was elected correspondent for the Section of General Physics of the Academy of Sciences on 25 November 1901. On 28 April 1913 he was made a nonresident member of the Academy.

Gouy was a prolific researcher, publishing dozens of articles in the major French scientific journals of his day. Most of his significant work was devoted to some of the more obscure problems of optics. By the 1870’s, when Gouy began his researches, optical theory had been subjected to a highly developed and fairly rigorous mathematical analysis, and it was felt that most of the major optical problems had been solved. By applying his talents to some of the less obvious areas of optical theory, Gouy was able to make contributions of considerable significance and originality.

Gouy’s first major optical paper, published in 1880, dealt with the velocity of light. He showed that in dispersive media it was necessary to distinguish between what is called the group velocity of light (the velocity of a series of light waves subject to direct measurement by J. B. Foucault’s method) and the somewhat higher and less easily measured velocity of the individual waves. Lord Rayleigh (John William Strutt) in a later, independent demonstration of Gouy’s theory labeled these two velocities the “group-speed” and the “wave-speed,” respectively. Both Gouy and Ralyeigh derived their results from mathematical theory. A.A. Michelson later confirmed them empirically, using carbon disulfide as a dispersive medium.

In another important paper concerning the propagation of light waves, Gouy demonstrated that spherical waves of weak emission at the same time, by a value that rapidly approaches one-quarter of a wavelength. An analogous consideration led him in 1890 to show that when spherical light waves are sent through the focus of a concave mirror, they “advance” by one-half a wavelength (in other words, the sign of their amplitude is reversed). This he demonstrated from a simple calculation derived from Christian Huygen’s analysis of point sources and also proved it experimentally. When two rays of white light are made to interfere with one another after being reflected from plane mirrors, the central fringe is always white. Gouy then showed that when one of these rays is reflected against a concave mirror, the central fringe is black because of the interference resulting from the reversal of the focus of the concave mirror.

Another area in which Gouy carried on extensive and original experimentation concerned the diffraction produced by the passage of light across the edge of an opaque screen. By concentrating his light on the border of the screen by means of a convergent lens and by using very thin screens with sharply defined edges, he was able to observe deviations of light rays at very large angles. His experimental results showed—contrary to what had been the accepted theory—that the nature of the screen played a large role in determining the degree of diffraction and the manner in which the light waves were polarized. The thickness of the screen and the material of which it was composed were especially important considerations in all experiments of this nature.

Although Gouy’s major achievements were in the realm of optics, he also devoted considerable attention to other areas of experimental and mathematical physics. He made important studies on the inductive powers of dielectrics, on electrocapillarity, and on the effects of a magnetic field on electrical discharge in rarefied gases. He was interested in spectroscopy. He investigated the emissive and absorptive powers of colored flames, and he invented a device to feed a constant supply of a salt to a flame in order to allow time to measure its spectroscopic emissions. The device made the air or gas take up the salt in the form of a fine spray before it reached the burner.

Gouy was also interested in phenomena produced by randomness of motion in nature. He was the first, perhaps, to point out that the nature of white light may best be understood as a complex disturbance resulting from a series of highly irregular impulses emanating from the source of light. The prism analyzes the irregular disturbance into constituents of definite wavelength, in the same way that a complex periodic function is analyzed mathematically into its simple harmonic components in a Fourier series. Gouy also made a detailed study of the randomness of Brownian movement in which he demonstrated that despite the irregularity of the movement, there is nevertheless a certain consistency independent of all adventitious circumstances.

Because of the number and variety of his researches, it is difficult to make a general evaluation of Gouy’s career. His name is associated with no physical law or theory. He made no important break-throughs; he opened no new areas of research. Great scientists work at the frontiers of knowledge; Gouy labored in the rear areas where most of the important work had already been accomplished. The significance of his researches was, in a sense, in tidying up the field of physical knowledge by extending already discovered theory into the obscurer areas that had been passed over in the first waves of discovery. His function was to integrate new phenomena into old theory, to extend and complete understanding rather than initiate it. If his work was not as significant as that of the great names in physics, it was nevertheless a vital and necessary part of the development of the science of his era.

BIBLIOGRAPHY

Gouy wrote no major works. A list of his many articles can be found in Poggendorff, IV, 520–521;V, 441–442; and VI, 933.

See the éloge of Gouy by Émile Picard in Comptes rendus hebdomadaires des séances de l’Académie des Sciences, 182 (1926), 293–295. Another notice by Picard was read to the Academy on 20 December 1937.

J. B. Gough