Why do clothes get darker when wet? © Getty Images

Why do clothes get darker when wet?

Asked by: Anna Daca (Leicester) and Calvin Tomsic (Everett, Washington, USA)

As water is transparent, it seems odd that it makes clothes look darker. After all, it doesn’t have that effect on, say, a hard plastic surface. Surprisingly, the science behind the phenomenon was only fully investigated around 30 years ago.

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Physicists John Lekner and Michael Dorf at the Victoria University of Wellington, New Zealand, showed that the darkening effect is the result of fabric being both rough and absorbent. When light strikes any surface, some of it is reflected back into our eyes.

But damp clothes have a thin layer of water on their surface (held in place by the material’s roughness), which leads to more of the reflected light rays being bent – ‘refracted’ – off-course. Some of the light also gets reflected back into the film of water, or scattered off the tiny water-filled holes in the fabric.

The combined effect is a reduction in the amount of light reaching our eyes, which makes the fabric look darker.

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