Why is it that when you get a white t-shirt wet, it goes almost completely transparent? Does the extra weight of the water stretch it thin or something? Just curious.
I’ve looked into a related question (although, considering my obsession with breasts, maybe I should’ve looked at this one first) – why does paper become almost transparent when you put oil on it? Back in the very earliest colonial days they actually used oiled paper for windows, but only for a short while (they soon switched to wet T-shirts. Then glass).
The secret to it all is index matching. Both the cotton fibers that make up a white T-shirt (assuming it’s uncolored) and the air are pretty transparent. The T-shirt appears white because you get reflection and scattering at each cotton/air interface. You can minimize this effect if you were to wet the cotton down with some liquid that has the same index of refraction as the cotton fibers. (The index of refraction is an optical constant that specifies how light is refracted upon entering the material. I(t also tells you the speed of light in that material). Even if the liquid doesn’t EXACTLY match the index of the cotton, it can still make the reflections smaller, rendering the shirt, if not completely transparent, at least translucent and (if in contact with what’s underneath) see-through.
That’s what happens with a wet T-shirt. The water, having an index of refraction of 1.333, is still somewhat lower than that of cotton (which is somewhere in the 1.4s, I think), but it’s enough to make the shirt see-through. But you don’t see perfectly – only the things that are in contact. Of course, because the shirt is wet, more is in contact. That’s the appeal of a wet T-shirt contest.
If you use oil on paper, the match is even better, but it can’t be perfect – in the first place, cotton fibers are birefringent, having different indices of refraction along different directions. In the second place, they put sizing in the paper to control the absorption of ink. They might have printing in either paper or T-shirts, and both may have fluorescent dyes added to make it appear “whiter”. All of this keeps either paper or a T-shirt from becoming completely transparent. Actually, even if these problems weren’t there, the paper or T-shirt wouldn’t become completely transparent, because it’s extremely hard to match refractive indices of liquids and solids over the entire visible spectrum. This leads to something called the Christianson effect, but I’m not going to get into that now.
ummm…I was lost after the “obsession with breasts” part…
Well, that’s the last time I try to edjikate you people.
Nice explanation Cal, but…
Shouldn’t that be ‘the Christiansen effect’, described as:
The monochromatic transparency effect produced by the immersion of a finely powdered substance (e.g., glass or quartz) into a liquid with a similar refractive index.
You know, only on the SDMB could a thread like this evolve in this way. 
I’m surprised you glossed over that brief yet unspeakably exciting time in late 1849 when some pioneers actually used breasts for windows, therefore expanding both their world view and the size of their “estates”.
Cal? You’re my kinda man.
Cartooniverse
Thanks, cal for the clarification. Now it’s perfectly clear to me! Actually, I think the matter needs much more research, so I’m soliciting funds to assist in this endeavor. Any sponsors?
Same reason you can’t see glass underwater, right? (I know it is. Rhetorical question. :))
mmm…wet t-shirt…
[sub]Note to self: get racinchikki a jug of water and a white t-shirt as a “welcome to the South” present.[/sub]
Cartooniverse:
Huge tracts of land, eh?
Whatever the cause, I’m just glad it does 
This is similar to the explanation as to why snow appears white. The white color you see is the reflection of the edges of the snowflake, even though the flake itself is essentially transparent ice.
OK Cal… (great answer by the way!) but why do nipples shine right through white t-shirts in black light?? Gotta love black light parties.
Where I work they’re about to spend $50K on a device to measure UV-visible fluorescence, so get me some volunteers in t-shirts (wouldn’t be scientific to use my OWN nipples)and some grant money and I’ll find out. Or try to, sort of. Get me the volunteers, anyway.
Not that this has anything whatsoever to do with the OP, but rayon ins transparent in the near infrared. So, if you have a near-IR camera…
Nevermind, forget I mentioned it.