Rayleigh, Lord

(John William Strutt). Born Nov. 12, 1842, at Langford Grove, county of Essex; died June 30, 1919, in Terling Place, near Witham. English physicist; one of the originators of the theory of vibrations. Fellow of the Royal Society of London (1873). Third Baron Rayleigh (1873).

Rayleigh graduated from Cambridge University in 1865. After the death of J. Maxwell in 1879, he became a professor at Cambridge and the director of the Cavendish Laboratory. In 1887 he was named a professor at the Royal Institution of Great Britain in London.

The range of Rayleigh’s scientific interests was very broad and included such fields of physics as acoustics, the theory of vibrations, optics, and electricity. He conducted extensive research on acoustic vibrations, for example, the vibrations of strings, rods, and plates. In 1873 he formulated a number of fundamental theorems of the linear theory of vibrations that permit qualitative conclusions to be drawn regarding the normal modes of vibration of vibratory systems, and he worked out a quantitative perturbation method for finding the normal modes of a vibratory system that differs little from a simple system with known normal modes. Rayleigh was the first to point out the special nature of nonlinear systems capable of undergoing continuous oscillation without a periodic input from without, and he was the first to draw attention to the particular character of such oscillation, which subsequently was called self-oscillation. He explained the difference between group and phase velocities; the formula he derived for the group velocity is now known as Rayleigh’s equation.

Rayleigh also considered the problem of adding many modes of random phases and derived the distribution function for the resulting amplitude—the Rayleigh distribution. The method worked out by him in this investigation governed the subsequent development of the theory of random processes for a long time. In the theory of elastic waves, Rayleigh studied sound pressure and the diffraction, scattering, and absorption of waves; he investigated waves of finite amplitude and the special type of surface wave called the Rayleigh wave.

Rayleigh’s research on the theory of vibrations was systematized in his fundamental work The Theory of Sound (two vols., 1877–78; 2nd ed., 1894–96), where for the first time there was clearly delineated a unified approach to the study of vibratory and wave processes that differ in nature. His ideas in this work constitute the foundation of the modern theory of vibrations.

In 1900, Rayleigh derived one of the laws governing the radiation of a blackbody—the Rayleigh-Jeans law. This work was of great importance to the development of quantum theory. Rayleigh laid the foundations for the theory of molecular light scattering. By establishing that the intensity of the scattered light is inversely proportional to the fourth power of the wavelength, he explained the blue color of the sky. In 1879, Rayleigh constructed a theory of the resolving power of optical instruments. In 1894, together with W. Ramsay, he discovered argon.

Many physical concepts, laws, and devices are named after Rayleigh. Examples are the Rayleigh disk, the Rayleigh interferometer, Rayleigh scattering of light, and Rayleigh’s law of magnetization. Rayleigh received a Nobel Prize in 1904.