Why paper is transparent when wet

Date: Tue, 9 Mar 2004 13:07:09 -0800 (PST)
From: Geoffrey M. Romer
To: East Chat
Subject: Re: ask Schmack

Why does cloth appear darker when it's wet?

Apparently the wet cloth becomes more transparent and less reflective.
It's not clear to me why wetness makes cloth transparent.

We covered this one in my class too. Mbrubeck's source is on the right track with the index of refraction business.

Cloth and sand and hair and paper and all the other materials that get dark when wet have the common property that (as the source notes) they are dielectrics, and so they're transparent. They also have the property of being composed of a whole bunch of tiny components, separated by air. In the case of cloth, cloth is made of threads and fibers, with air in between.

When light illuminates dry cloth, a typical light beam will hit a fiber, refract into it, and then refract back out of it. Because cloth has a high index of refraction, the light usually gets bent a lot at both refractions (remember, Snell's law says that the greater the difference between the indices of refraction, the greater the change in the angle of the light), and so it comes out in a different direction than it went in. Also, because cloth is not perfectly transparent, the light comes out a little dimmer than it went in. A typical light beam will undergo this process many times, passing through many fibers in succession, and so effectively, a lot of the light is scattered off the cloth in a more or less random direction. Some of this light hits your eye, which is how you can see the cloth.

When you get the cloth wet, the air between the fibers is replaced by water. Water has a much higher index of refraction than air, and so the relative index of refraction between water and the fiber is much lower than between air and the fiber. Thus, each time a light beam enters a fiber, it is bent much less than it would be with dry cloth. Thus, on average, the light beam has to pass through many more fibers before it is turned all the way around, and can come back out of the cloth to hit our eyes. Remember, though, that each time the light beam passes through a fiber, it gets dimmer, in a way that *doesn't* depend on the index of refraction. Since the light is now passing through a lot more fibers, it gets a lot dimmer before it leaves the cloth, so a lot less light is coming back to our eyes. Hence, the cloth appears darker.

This is a lot easier to explain with a blackboard, but my ASCII art skills are somewhat lacking, so I guess words will have to do. I can make this argument more formal, in terms of transport theory, but I suspect you folks don't care.

As is probably obvious, I love this stuff. I'm in grad school studying graphics specifically so that I can spend the rest of my life asking and answering these sorts of questions.

If you're shining a light onto the cloth, this makes it darker because it is reflecting less light back to you. If you shine a light through the cloth instaed, it's lighter when wet because it is more transparent.

caused by loose fibers being bound by surface tension. This person suggests that the liquid changes the cloth's index of refraction:


Related: The history of see-through bikinis (NSFW)

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