This is not about that. My question is “Why do wet garments sometimes look darn near transparent, and how can we make that happen more often?” Fans of Flashdance and the first three minutes of The Deep want to know. With certain fabrics, and certain lighting, a person might as well be nekkid. Yes, I’m not talking about naked, I mean nekkid! (don’t have no clothes on, and you up to something --Lewis Grizzard)
Basically, you’re doing a rough index matching, but it’s not perfect, so you only manage tanslucence (and you can’t get perfect matching with cellulose fibers, because they have different indices along different axes). But if you’re in direct contact with the underlying objects (as in a wet T-shirt contest), it’s almost as good.
It seems to me that Cecil’s answer can largely answer this question too. In this case, the light from the body more easily penetrates the fabric because the fabric is wet. But, unlike the case discussed by Cecil, fabric is thin, allowing the light to exit the other side (i.e., the side of the viewer) before being absorbed. So the light from the body can reach our eyes more easily when the fabric is wet. (In addition, more light from the room can reach the body, thus increasing the amount of light coming from the body.)
Also, the fact that wet fabric clings to the body helps because less light will reflect off the inside of the fabric. That is, the cling removes the pesky air-fabric interface that can reflect light back to the body. Moreover, we get to see the shape of the body more clearly (which, if nothing else, is bound to help us to imagine the body underneath).
Going back to Cecil’s answer, I’ve wondered if a second effect has much influence at making wet objects darker. Consider dry sand vrs wet sand. If the index of refraction of water is closer to that of each sand grain than that between air and sand, then won’t a greater fraction of light penetrate into each grain?
Cecil’s answer to the original question seems to be the same (though less detailed) as that described by Craig Bohren in the chapter “Multiple scattering at the beach” of the book “Clouds in a glass of beer”. Craig writes that placing a single grain of sand in various transparent fluids (e.g., air or water) does not ‘appreciably’ change the amount of light it absorbs. I wonder about this ‘appreciably’ part. It seems like a real messy calculation, but maybe someone has the answer?
