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The sky is blue because the atmosphere scatters the shorter wavelength (blue) end of the visible light spectrum more than the red end. As a result, more of this reaches the ground when the sun is high in the sky. Why then aren鈥檛 we aware of being bathed in a bluish light? Or do we in fact see a bluer world, but, because we always see it this way, our senses accept it as neutral? And is that why a tinge of blue is often added, for instance to washing detergent, to make things look whiter?

Philip Cunliffe, Bristol, UK

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This article appeared in print under the headline 鈥淥ut of the blue鈥�