Science Puzzle
Why Does a Wet T-Shirt Look Darker?
Spill water on a light-coloured T-shirt and the wet patch looks noticeably darker. The dye hasn't changed and the water is perfectly clear, yet the colour looks deeper. It's the same with wet sand on a beach, or a path after the rain.
Why does wet fabric look darker than dry fabric?
The Answer
Dry fabric bounces lots of light straight back at you. Wet fabric lets light travel deeper, where more of it is absorbed, so less comes back to your eye.
Light bends and partly reflects every time it crosses from one transparent material into another that slows it differently. Dry fabric is a tangle of fibres with air in all the gaps, so light entering it crosses fibre-to-air boundaries again and again. At each one a little light is reflected, and after many of these bounces a lot of it has been sent back out without going deep. That scattered light makes dry fabric look pale and bright.
Water slows light far more like the fibres do than air does. Fill the gaps with water and the boundaries almost vanish as far as the light is concerned, so far less is reflected at each one. The light goes deeper into the fabric and has more chances to be absorbed by the dye, so less returns: the patch looks darker, and its colour looks richer.
Crushed ice is white for the same reason: all those surfaces between ice and air scatter light back. A solid ice cube has very few, so it looks clear. Frosted glass becomes almost see-through if you stick clear tape over it, because the tape fills in the tiny bumps.
The principle: Scattering at boundaries. Light reflects wherever it crosses between materials that slow it by different amounts, so filling air gaps with water reduces scattering, lets light be absorbed deeper in and makes the material look darker.