John William Strutt, 3rd Baron Rayleigh

John William Strutt, who inherited the title of Lord Rayleigh in 1873, was the systematiser-in-chief of nineteenth-century wave physics. His two-volume Theory of Sound (1877-78) gathered every thread of classical acoustics — standing waves, resonators, damping, propagation in pipes and rooms — into a single coherent treatment that remained the standard reference for a century. It is in the Theory of Sound that the modern derivation of group velocity appears, extending Hamilton's 1839 identity with explicit attention to what happens when dispersion is present.

Rayleigh's contributions spread across almost every branch of classical physics. He explained why the sky is blue by showing that short-wavelength light scatters more strongly off air molecules than long-wavelength light — the phenomenon now called Rayleigh scattering. He co-discovered argon with William Ramsay in 1894, work that won him the Nobel Prize in Physics in 1904. He derived the Rayleigh-Jeans law of blackbody radiation, whose failure at short wavelengths pointed Planck toward quantum theory. He worked out the stability of fluid jets, the surface-wave modes that carry earthquakes, and half a dozen instruments that still bear his name.

He was, by all accounts, a relentlessly careful experimentalist who did most of his serious work in a private laboratory on the family estate at Terling Place. Einstein, asked late in life which physicists he most admired, named Rayleigh among the first three.