John William Strutt

John William Strutt, 3rd Baron Rayleigh (1842–1919), was an English physicist whose work in acoustics and optics helped establish modern physics. With William Ramsay, he discovered argon.

Rayleigh scattering: The scattering of light by particles much smaller than its wavelength, with intensity strongly dependent on wavelength. His analysis explained why the clear sky appears blue.
Rayleigh criterion: A rule estimating when two point sources can be distinguished by a diffraction-limited optical instrument. It expresses the resolution limit derived from his analysis of optical diffraction.
University of Cambridge: A public research university in Cambridge, England, founded in 1209. Strutt studied mathematics there and later held the Cavendish Professorship of Physics.
William Ramsay: A Scottish chemist who discovered several noble gases and received the 1904 Nobel Prize in Chemistry. He and Strutt isolated argon through complementary physical and chemical investigations.
Quantum mechanics: The physical theory describing matter and energy at atomic and subatomic scales. Quantum theory corrected the short-wavelength failure of the classical radiation law associated with his name.
Argon: A colorless, odorless noble gas that makes up about 0.93% of Earth’s atmosphere. Its discovery grew from his measurements showing atmospheric nitrogen was unexpectedly dense.
Rayleigh quotient: A scalar expression that estimates eigenvalues of a matrix or self-adjoint operator from a trial vector. This variational tool bears his name and reflects his use of energy methods in mathematical physics.
Cavendish Laboratory: The physics department of the University of Cambridge, established in 1874. He directed the laboratory from 1879 to 1884, succeeding James Clerk Maxwell.
J. J. Thomson: An English physicist who discovered the electron in 1897 through experiments with cathode rays. Thomson succeeded Strutt as Cavendish Professor and continued the laboratory’s experimental legacy.
Ultraviolet catastrophe: The contradiction between classical blackbody predictions and the observed finite energy emitted at short wavelengths. It exposed the limit of the Rayleigh–Jeans law and prompted Planck’s quantum hypothesis.

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