John William Strutt, 3rd Baron Rayleigh (/ˈreɪli/; 12 November 1842 – 30 June 1919), was an English physicist and mathematician.

Among many honours, he received the Nobel Prize in Physics in 1904 "for his investigations of the densities of the most important gases and for his discovery of argon in connection with these studies".

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).

Rayleigh was the second Cavendish Professor of Physics at the University of Cambridge (following James Clerk Maxwell), from 1879 to 1884.

On this post he carefully and precisely measured atomic mass of oxygen and hydrogen, and by 1892 he moved on to nitrogen.

He received the degree of Doctor mathematicae (honoris causa) from the Royal Frederick University on 6 September 1902, when they celebrated the centennial of the birth of mathematician Niels Henrik Abel.

[10][11] In 1904 he was awarded the Nobel Prize for Physics "for his investigations of the densities of the most important gases and for his discovery of argon in connection with these studies".

During the First World War, he was president of the government's Advisory Committee for Aeronautics, which was located at the National Physical Laboratory, and chaired by Richard Glazebrook.

[13] As an advocate that simplicity and theory be part of the scientific method, Rayleigh argued for the principle of similitude.

[15] When his scientific papers were to be published in a collection by the Cambridge University Press, Strutt wanted to include a quotation from the Bible, but he was discouraged from doing so, as he later reported: When I was bringing out my Scientific Papers I proposed a motto from the Psalms, "The Works of the Lord are great, sought out of all them that have pleasure therein."

In a letter to a family member, he wrote about his rejection of materialism and spoke of Jesus Christ as a moral teacher: I have never thought the materialist view possible, and I look to a power beyond what we see, and to a life in which we may at least hope to take part.

What is more, I think that Christ and indeed other spiritually gifted men see further and truer than I do, and I wish to follow them as far as I can.He held an interest in parapsychology and was an early member of the Society for Psychical Research (SPR).