Why does white clothing become see-through when wet?

We posed this question to Jeremy Baumberg from the Nano-photonic Centre in Cambridge...

Jeremy - Well let's talk about it the other way - how come we can't see through people's clothes? There are two reasons for that. One of them is because we put dyes in them, and these absorb certain colours of light and let other ones reflect back. But that's not true for white clothing, like cotton. So how come we can't always see through people's clothing to their underwear? The reason is because clothes are made of fibres which scatter light, and they scatter light in exactly the same way that milk looks white. It's got tiny particles called casein, which are about the same size as the wavelength of light, and light really strongly scatters against those particles into all directions, so we can't see through milk. So, cotton is made of lots of fibres around the same size as the wavelength of light, and we can't see through it. When it gets wet, there's water around all those fibres and then the light no longer gets scattered very strongly. So basically the material becomes more transparent.

Diana - But what about lifting the t-shirt away from the skin? Why should an extra layer of air make a difference?

Jeremy - What's happening is that each of the interfaces between the materials like the cotton and the water, and the air, light is getting bounced. It gets scattered around, and so, the fewer interfaces you have, then the less light gets scattered, and the more you can see through. So we might think of a nice experiment, how can we actually make clothes more transparent? Certain people might be interested in that. So you could imagine vapours with liquids which would absorb onto the cotton fibres. The fibres swell as well, that's also why the interface is changed. So you can imagine some very devious scientists, deciding to really make clothes more transparent, with the right spray.