Robert John Strutt, 4th Baron Rayleigh FRS[1] (28 August 1875 – 13 December 1947) was a British peer and physicist.

Strutt was born at Terling Place, the family home near Witham, Essex, the eldest son of John William Strutt, 3rd Baron Rayleigh and his wife Evelyn Georgiana Mary (née Balfour).

[2] He became a research student in physics at the Cavendish Laboratory under J. J. Thomson, whose biography he subsequently wrote.

He wrote one of the first books on radioactivity, The Becquerel rays and the properties of radium (E. Arnold, 1904).

As one who has made discoveries in Physics and as the author of the following papers: – 'On the Least Potential Difference required to Produce Discharge through Various Gases' (Phil Trans, vol cxciii, 1893); 'The Dispersion of the Cathode Rays by Magnetic Gases' (Phil Mag,, Nov 1899); 'The Discharge of Electricity through Argon and helium' (ibid, March 1900); 'The Behaviour of Becquerel and Rontgen Rays in a Magnetic Field' (Proc Roy Soc, vol lxvi); 'The Conductivity of Gases under Becquerel Rays' (Phil Trans, vol cxcvi, 1901); 'The Tendency of the Atomic Weights to Approximate to Whole Numbers' (Phil Mag,, March 1901); 'The Discharge of Positive Electrification by Hot Metals' (ibid, July 1902); 'Electrical Conductivity of Metals and their Vapours' (ibid, Nov 1902); 'Some Recent Investigations on Electrical Conduction' (Proc Roy Inst, April 1903); 'Preparation and Properties of an Intensely Radio-active Gas from Metallic Mercury' (Phil Mag, July 1903); 'Radio-activity of Ordinary Materials' (ibid, June 1903); 'Absorption of Light by Mercury and its Vapour' (ibid, July 1903); 'The Intensely Penetrating Rays of radium' (Proc Roy Soc, lxxii); 'Fluorescence of Crystals under Rontgen Rays' (Phil Mag, Aug 1903); 'An Experiment to Exhibit the Loss of Negative Electricity by Radium' (ibid, Nov 1903).He delivered their Bakerian Lecture in 1911 and 1919.

[6] Strutt's best known work in the period 1904–1910 was the estimation of the age of minerals and rocks by measurement of their radium and helium content.

In 1916, working with his colleague Alfred Fowler, Strutt was the first to prove the existence of ozone in the atmosphere by examining the ultra-violet spectrum of the setting sun.

The importance of his unpublished data was such that the US Airforce Cambridge Research Laboratories acquired it in 1963, almost by accident, at the same time of many of his father's experimental notebooks.