Robert Strutt, 4th Baron Rayleigh

Robert John Strutt, 4th Baron Rayleigh (28 August 1875 – 13 December 1947), was a British peer and physicist known for his contributions to the study of gases and atmospheric phenomena.

**Early Life and Education**

Strutt was born at Terling Place, the family residence near Witham, Essex. He was the eldest son of John William Strutt, 3rd Baron Rayleigh, and Evelyn Georgiana Mary (née Balfour). His familial relations include nephews of Arthur Balfour and Eleanor Mildred Sidgwick. He attended Eton College before matriculating at Trinity College, Cambridge. Initially studying mathematics, he shifted after two terms to the Natural Sciences tripos. Subsequently, he became a research student in physics at the Cavendish Laboratory, working under J. J. Thomson, whose biography Strutt later authored. His early research focused on electrical discharge in gases, including studies related to X-rays and electrons. In 1904, he authored one of the first books on radioactivity titled *The Becquerel Rays and the Properties of Radium*. He was awarded the Coutts Trotter studentship in 1898 and was a Fellow of Trinity College from 1900 to 1906. He received his Master of Arts degree in 1901.

**Academic and Professional Career**

In May 1905, Strutt was elected a Fellow of the Royal Society, with his candidature citing numerous discoveries in physics and a substantial list of published papers. His work addressed topics such as electrical discharge in gases, the behavior of cathode rays, radioactivity, and fluorescence under X-ray illumination. He delivered the Royal Society's Bakerian Lecture in 1911 and 1919 and served as president of the British Association for the year 1937–1938.

From 1904 to 1910, Strutt's research chiefly involved estimating the age of minerals and rocks via radium and helium measurements. In 1908, he was appointed Professor of Physics at Imperial College, London. During this tenure, he built upon his father’s work on light scattering, now known as Rayleigh scattering, publishing several papers on the subject.

**Scientific Contributions**

In 1910, Strutt discovered that an electrical discharge through nitrogen gas produced what he termed "active nitrogen," an allotrope thought to be monatomic. The glowing cloud produced by this apparatus could react with quicksilver to form explosive mercury nitride. In collaboration with Alfred Fowler, he was the first to confirm the presence of ozone in Earth's atmosphere by analyzing its ultraviolet spectrum during sunset in 1916. He demonstrated that ozone was predominantly located in the upper atmosphere, in a region now referred to as the ozone layer.

After his father's death in 1919, Strutt resigned from his academic position but continued experiments in his private laboratory. His earlier work on gaseous discharge and fluorescence led to further studies on the luminosity of the night sky. He was the first to distinguish between auroras and airglow, the latter preventing the night sky from appearing completely dark globally. In 1929, he was the first to measure the intensity of night sky light, a subject that earned him the nickname "the Airglow Rayleigh." His unpublished data concerning airglow was acquired by the US Air Force Cambridge Research Laboratories in 1963 and is now preserved at the US Air Force Academy. The unit "rayleigh," a measure of photon flux related to airglow, is named in his honor. A special edition of *Applied Optics* published in 1964 dedicated pages to the 3rd and 4th Barons Rayleigh.

**Later Life and Personal Details**

Strutt inherited the title of 4th Baron Rayleigh upon his father’s death in 1919. He married twice: first to Lady Mary Hilda Clements on 5 July 1905 (she died in 1919), and then to Kathleen Alice in 1920. He had a total of six children—five with his first wife, including his heir John Arthur Strutt, 5th Baron Rayleigh, and one with his second wife.

His children include:

- Violet Blanche Strutt (1906–1910)

- John Arthur Strutt, 5th Baron Rayleigh (1908–1988)

- Hon. Charles Richard Strutt (1910–1981)

- Hon. Daphne Strutt (1911–2003)

- Hon. Hedley Vicars Strutt (1915–2012)

- Hon. Guy Robert Strutt (1921–2007)

He died in Terling, Essex, in 1947.