BLUE SKIES

There is something deeply human about wondering why the unclouded portions of a daytime sky are blue. The tradition of trying to ascertain the reason for the phenomenon dates back to at least the Aristotelian treatise On Colours. Those people who have examined the question have included: Roger Bacon, Leonardo da Vinci, Johannes Kepler, Ren Descartes, Sir Isaac Newton, Leonhard Euler, and Horace-Benedict de Saussure.

John Tyndall held the Professorship of Natural Philosophy at The Royal Institution. One of the topics that he investigated is what is now known as long-wave infrared radiation. In 1859 he announced the Tyndall Effect. This describes how a light beam that is shined into a clear fluid is scattered by the particles of matter that it encounters in its path. A dozen years later the aristocrat and academic physicist John Strutt published two papers that were entitled On The Light From The Sky, Its Polarisation and Colour and On The Scattering of Light By Small Particles. These addressed the matter in greater detail and furnished a mathematical description for the atmospheric phenomenon. The non-blue portions of the spectrum have longer wavelengths than the blue ones possess. Therefore, the former are able to pass through the gas molecules that are present in the ether. Whereas, the latter strikes these and are reflected by them (in the Rayleigh Scattering) thereby creating the impression that the sky is blue.

Strutt s work utilised the accepted elastic solid theory of light. It did not draw upon the ideas that James Clerk Maxwell was propounding. However, the former s line of argument did not prevent the latter from regarding the man s ideas as being substantive. In 1873 Maxwell wrote to the now 3^rd Baron Rayleigh, who by then had inherited his family title, to urge him to investigate the problem further. Intermittently, the peer spent time working on the matter.

Rayleigh published a polished version of his theory in 1899 in an article that he entitled On The Transmission of Light Through An Atmosphere Containing Small Particles In Suspension, and On The Origin of The Blue of The Sky. In it, he maintained his erroneous opinion that the blue light was striking water vapour and dust in the atmosphere. The paper did not resolve every aspect of the subject. Albert Einstein became interested in the topic. He drew upon the work of the Polish physicist Marian Smoluchowski and in 1911 published a piece that explained those aspects of it that the baron had not settled. He contended that the azure derived from the blue light s interaction with oxygen and nitrogen.

Location: 4 Carlton Gardens, SW1Y 5AD. The Conservative politician Arthur Balfour was the leaseholder. He and Rayleigh had become close friends while they had been students at the University of Cambridge. The scientist had married the future premier younger sister Evelyn. Britain experienced a sustained agricultural depression at the end of the 19^thC. This caused the baron s family finances to be (relatively) straitened. Therefore, he and his wife were the houseguests of her sib whenever they spent time in London over the period 1878 to 1897. (blue, orange)

The Royal Institution, 21 Albemarle Street, W1S 4BS (red, brown)

Website: www.rigb.org/explore-science/explore/collection/john-tyndalls-blue-sky-apparatus www.phy.cam.ac.uk (Rayleigh s financial circumstances caused him to hold the University of Cambridge s Cavendish chair of physics from 1879 to 1884. Maxwell had preceded him in the position.)

David Backhouse 2024