John William Strutt, Lord Rayleigh (b. November 12, 1842, Langford Grove, Maldon, England – d. June 30, 1919, Terling Place, Witham) was a physical scientist who made fundamental discoveries in the fields of acoustics and optics that are basic to the theory of wave propagation in fluids. He received the Nobel Prize for physics in 1904 for his successful isolation of argon, a noble gas appearing in the Earth's atmosphere.
Strutt graduated from Trinity college, University of Cambridge, in 1865 as Senior Wrangler (highest score) in the Mathematical Tripos. Further he won the Smith's Prize (a prestigious competitive award for an essay that incorporates original research). His first paper (1869) explained the electromagnetic theory of James Clerk Maxwell in terms of analogies that a non-physicist would understand. In 1873 he became the third Baron Rayleigh, on the death of his father, the second Baron. In the same year he became Fellow of the Royal Society of London.
On an excursion to Egypt taken because of his health, he began writing his famous two-volume treatise The Theory of Sound, published in 1877–78. He served as the second Cavendish professor of experimental physics (1879–84) at Cambridge, succeeding Maxwell. Succeeding Tyndall, he served as Professor of Natural Philosophy at the Royal Institution from 1887 to 1905. From 1885 to 1896 he was secretary of the Royal Society of London and in 1905 he became president, which he stayed until 1908. In that year he became chancellor of Cambridge University.
During the 1880s he conducted precision measurements of the density of gases. After noting that the density of the nitrogen gas isolated from the atmosphere was somewhat higher than the density of nitrogen obtained by chemical means, he isolated argon in 1895 from atmospheric nitrogen, thus explaining its higher density by the presence of small amounts of argon.