I’ll assume that you’re in the dining room, concerned about soup on your necktie, although you may have noticed this phenomenon in other rooms under different circumstances.
We see an object because light is coming from that object and entering our eyes. The more light coming from the object, the brighter it appears. And of course, the reverse is also true: An object that is sending less light to our eyes appears darker. So our job is to explain why there is less light coming from the wet spot.
Where does an object get the light that it sends to our eyes? If it is not inherently luminous, like the sun, a light bulb or Rudolph’s nose, then it must be reflecting some of the light that it receives from elsewhere. But nothing reflects all of the light that falls upon it; every substance absorbs some light and returns, or reflects, the rest. So the wet spot must be reflecting less light because for some reason it is absorbing more.
Let’s take a highly magnified look at the wet fabric as it would be seen by an incoming ray of light.
A fabric is a latticework of interwoven fibers. When it gets wet and soaks up water by capillary action, the spaces between the fibers become filled with water. Many of the incoming rays of light will then be falling upon a water surface instead of striking a fiber.
Now when a ray of light enters a water surface at an angle, and by sheer statistics most of the rays will be hitting the water at an angle, rather than perfectly perpendicular to its surface, a funny thing happens: The ray changes direction. (Techspeak: It is refracted. Why does it change direction? Meet me in the Nitpicker’s Corner.)
Instead of continuing through the water in the direction in which it entered, the light ray veers away from the surface and plunges into the watery depths at an even steeper angle than its entry angle. This steeper angle of penetration means that the light ray penetrates deeper into the depths of the fabric, where it has an increased chance of being absorbed, never to be seen again. Thus, there is more “lost light” inside a wet spot than in a dry one, there is less light reflected and the spot appears darker.
Similar goings-on explain why wet rocks, leaves and grass appear to be more intensely colored when they’re wet, why the countryside looks “fresher” after a rain. These objects have colors in the first place because they absorb certain wavelengths of light from the multicolored daylight and reflect the rest back to our eyes. When they are coated with a film of water, the incident light rays are refracted deeper into their microscopically rough surfaces.
The refracted light then bounces back and forth off these surfaces, which provides them with many more opportunities for their absorbable wavelengths to be absorbed. The remaining reflected light is thus even more depleted in these absorbed wavelengths than it ordinarily would be, and it therefore looks more intensely colored.