LS Answers: Why is the sky blue?

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Look to the daytime sky in clear (cloud-free) conditions and you’ll see that it is light blue in colour. Until around 150 years ago, a sound scientific explanation as to why the sky was blue had not been given. In 1859, Irish physicist John Tyndall provided an answer to this age-old question by noting the changing colour of a clear fluid holding small particles in suspension, when viewed from different angles. The term Tyndall effect (or Tyndall scattering) is used to describe this behaviour, the same behaviour that explains why the sky is blue.

Visible light is part of the spectrum of radiation emitted by the sun. This light looks white, but it’s actually made up of a spectrum of colours with different wavelengths. As this radiation travels through Earth’s atmosphere, some of it is scattered by the molecules of gas that make up our atmosphere. However, not all wavelengths in the visible spectrum scatter equally. Blue light, at one end of this spectrum, scatters more than red light – at the other end of the spectrum – which passes through the atmosphere with less scattering. The reason for this is due to blue light having a smaller wavelength (higher energy). Hence, with blue light being scattered around the sky much more efficiently than other colours, the sky appears blue.

Those familiar with the electromagnetic spectrum may ask: ‘Why then, does the sky not appear violet, or indigo?’ on account of the fact that it is these colours that have the shortest wavelength in the visible spectrum, and so should scatter more readily. The answer is two-fold. First, some violet light is absorbed by the high atmosphere, and so there is less reaching our eyes. The second reason involves the way our eyes perceive different colours.  We have three types of colour receptors (or cones) in our retinas, called red, green, and blue – because they respond most strongly to light at these wavelengths. However, each cone corresponds to a range of wavelengths so that – for example – the blue cone is stimulated by all colours near the blue wavelengths, such as indigo and violet. In effect, we actually interpret the blue-violet light of the sky as a mixture of blue and white light, so that overall the sky appears light blue.

Dom Salisbury

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