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It's a well-known fact that when one soaks a thin piece of fabric, it will often become more transparent than it was before.

What is the reason behind this? I can't put glass behind the fabric and increase its transparency.

(Also, just in case this seems like a duplicate, I am not concerned necessarily with why the fabric become darker but just the transparency aspect, unless, of course, the causes of these two phenomena are not disjoint.)

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The fabric is made of many thin fibers of fabric, with air in between. This structure causes light to bounce around many times inside, making it hard for light to get through.

When you make the fabric wet, you replace the air with water, which has a closer index of refraction to the fibers. So the reflections inside are less important, and more light just goes straight through.

Some people might interpret this as the clothing getting darker, especially if somebody is wearing the clothes, but that’s just because no light is coming from the other side. If you hold the fabric up to the sun, it should be clear the effect is really just an increase in transparency.

For the same reason, paper becomes transparent when it gets oily, as anybody who has ever eaten pizza on a paper plate knows.

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    $\begingroup$ Indeed; see greased paper window. $\endgroup$ – Chemomechanics May 26 '18 at 20:08
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    $\begingroup$ One might also find that dry fibers are fuzzier (present a larger geometric obstruction) than are wet fibers. (I would have thought a side-by-side magnified image of the same thread wet and dry would be easy to find on the 'net, but it turns out I have weak Google-fu.) $\endgroup$ – Eric Towers May 26 '18 at 21:59
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    $\begingroup$ trying to think of an example where thin fabric becomes more transparent in real life... $\endgroup$ – MarsJarsGuitars-n-Chars May 27 '18 at 18:45
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    $\begingroup$ "Some people might interpret this as the clothing getting darker." Something transparent will allow more light to pass through, and therefore reflect less light. This seems a lot like actually being darker rather than just seeming darker. $\endgroup$ – BobTheAverage May 28 '18 at 15:21
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    $\begingroup$ @mbrig I happen to have a can of air sitting next to me ("contains difluoroethane", don't breath this!) and...yes, it does in fact make paper transparent and 'dries' without a trace. The stuff I am certain I saw a YT video on (e.g. a channel like Cody's Lab) lasted longer. $\endgroup$ – Draco18s May 28 '18 at 17:44

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