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Tell me why the stars do shine 
Tell me why the ivy twines 
Tell me why the sky’s so blue 
And then I’ll tell you just why I love you…

Well, if you could have sung this little tune to the Irish physicist John Tyndall, born on this day back in 1820, he would have had some strong opinions, at least on the blue sky question.

In addition to many other achievements, Tyndall published studies on acoustic properties of the atmosphere and the blue colour of the sky – he suggested the colour was the result of the scattering of light by small water particles. He discovered that, when light passes through a substance containing small suspended particles, the shorter wavelengths (blue side of the spectrum) are scattered more than the longer, red wavelengths. Since the blue light is scattered in all directions, the substance appears blue.

This phenomenon became known as the Tyndall effect.

What we see as a lovely blue sky, John Tyndall saw as a scientific challenge.
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Thus, a clear day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. Towards sunset, when we look towards the sun, we see reddish colours, because the blue light has been scattered away from the line of sight.

The Tyndall effect also causes other interesting blue colourings in nature, including blue eyes, opalescent gemstones and the wings of some birds and butterflies. When colour is caused by scattering of light it is known as a structural colour, as opposed to a pigment colour.

Now about those stars and ivy…

(Source: Why is the sky blue?)