John Strutt, 3rd Baron Rayleigh

John William Strutt, 3rd Baron Rayleigh (/ˈreɪli/ RAY-lee; 12 November 1842 – 30 June 1919), was a British physicist and hereditary peer who received the Nobel Prize in Physics in 1904 for his discovery of argon.

Rayleigh provided the first theoretical treatment of the elastic scattering of light by particles much smaller than the light's wavelength, a phenomenon now known as Rayleigh scattering, which notably explains why the sky is blue. He studied and described transverse surface waves in solids, now known as Rayleigh waves. He contributed extensively to fluid dynamics, with concepts such as the Rayleigh number (a dimensionless number associated with natural convection), Rayleigh flow, the Rayleigh–Taylor instability, and Rayleigh's criterion for the stability of Taylor–Couette flow.

Rayleigh also formulated the circulation theory of aerodynamic lift. In optics, he proposed a well-known criterion for angular resolution. His derivation of the Rayleigh–Jeans law for classical black-body radiation later played an important role in the birth of quantum mechanics (see ultraviolet catastrophe). His textbook, The Theory of Sound (1877), is still used today by acousticians and engineers. He introduced the Rayleigh test for circular non-uniformity, which the Rayleigh plot visualises.

Rayleigh served as President of the Royal Society from 1905 to 1908, and as Chancellor of the University of Cambridge from 1908 until his death in 1919.