Why Are Sweaty-Armpit Stains So Dark?

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Why is fabric darker when it's wet?
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▼ No matter how much deodorant we use, it's challenging to hide the signs of overexertion. Armpit stains and back perspiration are all too visible under the fluorescent lights at the local gym, as our T-shirts and tank tops take on a two-tone shade of sweatyand dry.
But why is fabric darker when it's wet? You might resent that clothing manufacturers haven't solved the issue yet. However, it has nothing to do with the makeup of the T-shirt itself. Rather, it's a matter of how the eyes perceive light and how some wet objects refract light differently than their dry counterparts.
In reality, wet fabric isn'tactuallydarker than dry fabric. Rather, it just looks darker to the human eye. The same goes for other wet-versus-dry surfaces, like wet cement after a rainstorm, or wet sand after waves slosh over it.
When light — from the sun or a lamp, for example — hits an object, some of that light is absorbed, and some is reflected. The wavelength of light (measured on a scale of billionths of meters) that's reflected off the object back into our eyes determines what color we perceive the object to be. A blue shirt absorbs all wavelengths of light except those that make up the color blue (roughly 450 nanometers) and reflects what's left into the area around it, including our light- sensitive retinas.
But according to Richard Sachleben, a retired chemist and member of the American Chemical Society's panel of experts, the colors we perceive in that reflected light depend on the properties of the surface the light is bouncing off.  (▪ ▪ ▪)

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