杏吧原创

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?

鈥� Rayleigh scattering explains why light from the sky is bluer than light from the sun as seen from outside Earth鈥檚 atmosphere, but it doesn鈥檛 explain why we 鈥減erceive鈥� it as blue. To explain that, you need to define 鈥渨hite鈥�.

Our vision evolved on Earth鈥檚 surface, where the average illumination is a mixture of bluish light from the sky and direct sunlight, which is slightly yellowish because some blue has been removed by Rayleigh scattering. That average illumination is close to the colour of the sun as seen from outside Earth鈥檚 atmosphere and is what we perceive to have no particular colour, in other words, 鈥渨hite鈥�.

On an overcast day we do, in fact, see the world as slightly blue.

If we were visited by astronauts from another planet whose star was bluer than ours, they would perceive our white as slightly yellow, or if their star was yellower than ours, they would perceive our white as slightly blue.

Richard Parkins, Via email, no address supplied

鈥� When the sun is in a clear blue sky, measurements show that only about 6 per cent of the illumination comes from the blue sky. Thus we perceive the world to be illuminated by the pure white light of the sun (and it really is white, not yellow, as the colour of clouds reveals: they only become yellow or red around dawn and dusk).

Objects in the shadows, however, are lit only by the sky鈥檚 light and so are tinted towards blue, as artists have always known. They commonly add a bluish tone to represent outdoor shadows, with a violet tinge often preferred by watercolourists. Shadows can be complex, though, because the sun鈥檚 bright white light is often reflected into the shadows from surroundings such as green trees, providing additional colouring of shaded areas.

The addition of dolly or laundry blue to white washing is a different matter. This was to counteract the natural yellowing of the white fabric, but its use has been superseded by the addition of blue fluorescence in the detergent, which is more effective.

John Elliott, Stockport, Cheshire, UK

鈥� We aren鈥檛 living in a wash of blue because the blue light from the sun is scattered in all directions, not just down towards the ground. Thus the sun appears yellow, and the overall light received at the ground is reduced in blue wavelengths. The effect becomes much more extreme towards sunset, because the blue wavelengths are scattered to an even greater extent due to the longer passage of light through air. That is why sunsets are red.

However, the questioner is correct in saying there is some colour-balance correction going on automatically in the brain, and that we naturally react to, and to some extent correct for, the changes in light frequency throughout the day.

This processing becomes apparent if you ever observe a total solar eclipse during the middle of the day. As totality approaches, the light becomes very dim, but because the sun hasn鈥檛 significantly changed its position, the light isn鈥檛 reddened as it would be at sunset. The effect seems very washed out and unnatural.

Tim Cutts, Milton, Cambridge, UK

We pay 拢25 for every answer published in New 杏吧原创. To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.

杏吧原创 retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.

You can also submit answers by post to: The Last Word, New 杏吧原创, 25 Bedford Street, London WC2E 9ES.

Terms and conditions apply.