Science Questions

Why do wet clothes become see-through?

Sun, 28th Feb 2010

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Question

John, Louisville asked:

I've had a burning question in the back of my mind since I went to a scientific meeting of ocular scientists down in Sarasota, Florida. There was this lovely young scientist from Budapest who forgot her bathing suit and put on shorts and a white T-shirt when she went swimming, and she was alarmed (and some of us, not so alarmed) to discover that the T-shirt became virtually transparent as soon as it got wet. But she realized this and she was also smart enough to pull the T-shirt away from her body when she did that. It was no longer transparent and I'm not really sure what the optics involved were and why that happens. Iíd be very interested if someone could offer an answer!

Answer

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

Wet t shirtJeremy -  Well letís talk about it the other way - how come we can't see through peopleís 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 thatís not true for white clothing, like cotton.  So how come we can't always see through peopleís 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.  Itís 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, thereís 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 -   Whatís 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, thatís 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.

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well I'm not entirely sure, nor am I an expert, but my intuition on the matter told me this: the water molecules hosted by the white cloth act like those glass prisms that were installed in the decks of ships to allow light to filter down to lower levels. That is to say, the water molecules act like so many tiny little prisms and provide a conduit for the light to find its way through the previously opaque cloth. Now, the reason you could only see what was under ;) the t-shirt when her skin was in contact with the cloth is simply that the water has made the shirt quite heavy and droopy, and as such tends to stick quite close to the body, affording the light a relatively miniscule distance to travel before reflecting off the skin pigment and journeying back through the cloth. I suspect this is enhanced by water's clinging properties due to surface tension, as a chain of water links the outside of the cloth with the bare skin directly.

sparknotes: the water allows light to bend around the cloth molecules, but not so much as to render it transparent when viewer or object aren't in direct contact with the barrier.

diagram:
grahamsteen, Fri, 26th Feb 2010

I think it has to do with refraction, and that some clothes like thin white cotton begets the same refraction index, more or less, as water when wet and so lets light in and out again. I'm also guessing that other colors might change it? yor_on, Mon, 1st Mar 2010

I think there are three major factors at play here:

1) Surface tension

2) Overactive imagination

3) Wishful thinking Geezer, Mon, 1st Mar 2010

well i have all three in abundance, so you're probably right. worth a shot, though. grahamsteen, Tue, 2nd Mar 2010

well i wasn't THAT over imaginative. verbose, maybe. grahamsteen, Tue, 2nd Mar 2010

I thought it was because the fibers and filaments of the cotton are running and spreading around when dry, but clump together around the main fibers when wet, creating larger gaps between the fibers and more of the ability to see through. The distance between interfaces seems more salient though, just like when you can see through a cloth that's right in front of your eyes. Adam, Mon, 14th Jun 2010

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