Who discovered why the sky is blue

What’s the scientific question that children most often ask their parents? Given that today we have surveys on almost everything, they also exist on this topic, and there does seem to be one single question that is consistently repeated: Why is the sky blue? The data show that the vast majority of parents struggle to respond to an enigma solved a century and a half ago by a curious Irishman: the physicist John Tyndall.

Born in the small town of Leighlinbridge, the son of a policeman father and a mother disinherited for marrying him, Tyndall (2 August 1820 – 4 December 1893) is one of those rare scientists whose name arises unexpectedly in the thread of such disparate subjects that one comes to wonder if it’s the same person. Whether the topic is anaesthesia, the greenhouse effect, the sterilization of food, the first climb of Mount Weisshorn, the principles of fibre optics or the mystery of magnetic levitation, the same name pops up: John Tyndall.

Formally, the Irish scientist launched his career as a reputed physicist thanks to his studies on diamagnetism, the repulsion on which superconductors or magnetic levitation trains are based. These works earned him the appreciation of Michael Faraday, who became his mentor. However, perhaps his most original contributions were made in the field of radiant energy—later called infrared—of gases.

Demonstrated the greenhouse effect

It was this line that led him to discover the high infrared absorption of water vapour, thus demonstrating the greenhouse effect of the terrestrial atmosphere that until then had only been speculation. But these studies also led him down a peculiar path: by inventing an apparatus that measured the amount of CO2 exhaled by the human breath by means of its infrared absorption, he laid the foundations of capnography, the system that is used today to monitor the breathing of anesthetized patients or those in intensive care.

This was not Tyndall’s only incursion into biomedicine, nor the one that prompted the University of Tübingen to award him an honorary doctorate of medicine. Half a century before Alexander Fleming, he was one among several scientists who independently studied the bactericidal properties of the fungus Penicillium. His interest in the air, his main study material, led him to conserve boiled broths in a germ-free atmosphere, an experiment that escaped Louis Pasteur. And when bacterial spores contaminated his broths, he invented tyndallisation, the first method of food sterilization that killed these heat-resistant forms.

While his enthusiasm for mountaineering led him to make the first successful ascent of the Weisshorn and to lead one of the first teams to reach the peak of the Matterhorn, he entertained himself by studying the dynamics of the glaciers. And between one occupation and another, he still had time to practice another great passion: science dissemination. His books are among the best pioneering examples of the popularisation of science for a non-specialist audience. In his lectures to crowded auditoriums in Britain and the US, he marvelled his viewers by showing how a ray of light was diverted by a stream of water, the principle on which modern fibre optics is based.

The enigma of the blue sky

And Tyndall explained why the sky is blue. He did so in the 1860s at the Royal Institution in London where he served as professor of physics for 34 years. In the course of his research on the radiant energy of the air, he constructed a glass tube that simulated the atmosphere, with a source of white light at one end that acted as the Sun. Tyndall observed that, as he introduced smoke into the tube, the beam of light appeared bluish when looking at the side of the tube, but reddish looking from the end opposite the light source.

This phenomenon led him to propose that the particles of dust and vapour in the atmosphere scattered the blue light that came into our eyes. Today we know that blue is scattered more due to its shorter wavelength, while red penetrates more because it is the longest wave of visible light. As the distance that light travels through the air increases, as at dawn and dusk with the Sun low on the horizon, the blue light is absorbed before reaching our line of sight and we observe the scattering of the red

Curiously, Tyndall was actually wrong. The so-called Tyndall effect describes this phenomenon of dispersion in fine-particle fluids, but what we see in the sky is actually the so-called Rayleigh scattering caused by the air molecules themselves, which are much smaller than the wavelength of light (and not by the much larger dust particles). However, this is a technicality that does not take away from the fact that parents all over the world should be grateful for Tyndall’s genius.

Javier Yanes

@yanes68

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