Why do wet clothes look darker when water is clear?

Filed under cases that solve themselves once you stop staring at the shirt and start following the light. Wet clothes look darker because less light makes it back out of the fabric to your eye — not because anything about the color changed. Water is clear, the dye is untouched; the darkness is a bookkeeping problem in reflected light.

Case notes: the dry shirt

A dry shirt is rougher than it looks. Every fiber is wrapped in tiny air pockets, and each fiber-to-air boundary is a little mirror. Light dives in, ricochets between thousands of these boundaries, and most of it gets flung right back out at you. All that returned light reads as bright — which is why a dry white shirt looks white instead of see-through, even though the individual fibers are mostly transparent.

The real suspect: the water film

Spill water on the shirt and it floods those air pockets. Water bends light much the way the fibers do — its refractive index (a number for how much a material bends light, about 1.33 for water) sits close to the fibers' and far from air's 1.00. The little mirrors effectively vanish. Light now slips deeper into the weave instead of bouncing straight back. Worse for the light, the smooth water film works like a one-way door: rays trying to exit at a shallow angle — past about 48.6 degrees — get reflected back inside. Each extra bounce is another chance for the dye to absorb them. More light absorbed and transmitted, less returned: the spot reads darker and the color richer.

Closing the file

The proof is in the drying. As water evaporates, the air pockets reopen, the tiny mirrors come back on duty, and the patch pales back to its old self. Nothing was stained and nothing was bleached; the fabric just briefly got better at swallowing light than at throwing it back.