John William Strutt, 3rd Baron Rayleigh (; 12 November 1842 – 30 June 1919) was an English physicist who, with William Ramsay, discovered argon, an achievement for which he earned the Nobel Prize for Physics in 1904. He also discovered the phenomenon now called Rayleigh scattering, which can be used to explain why the sky is blue, and predicted the existence of the surface waves now known as Rayleigh waves. Rayleigh's textbook, The Theory of Sound, is still referred to by acoustic engineers today.