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Why is water colourless?

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Or should we wonder whether it is actually colourless at all?

Asked by: Wael Alsallami, Yemen


Lab measurements show that water does have a colour: pale blue. Given the blue colour of the sea, that may come as little surprise. But according to Dr Martin Chaplin, an expert on the properties of water, its colour has a specific cause.

Its origins lie in the way the H20 molecule interacts with incoming light. The molecule’s two hydrogen atoms sit at the ends of two spring-like ‘legs’ joined midway by the oxygen atom. The resulting V-shaped combination can vibrate in various ways, mopping up different wavelengths of light. But it’s particularly effective at absorbing longer, redder wavelengths, while leaving shorter, bluer wavelengths fairly untouched. The result is a pale blue colour.

According to Chaplin, while this absorption contributes to the sea’s colour, there’s scattering at work too. Water scatters shorter wavelengths more effectively, leading to more of the blue component of sunlight reaching our eyes.


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Robert is a science writer and visiting professor of science at Aston University.


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