Why does clothing appear darker when it is wet, even though water is transparent?
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If you were to zoom way in on the fibers of clothes or paper, you would see that they’re pretty reflective. That’s because they’re pretty much transparent. Everything that’s transparent has something called a refractive index, and the more different two refractive indexes are, the more they will look reflective instead of invisible. The plastic or cotton fibers have a very high refractive index, and air is very low, meaning that your clothes reflect most light.
Water on the other hand has a refractive index between the two. That means when your clothes get wet, the light will go into the water without being reflected much, then into the fibers without being reflected much.
In other words, they becomes darker because they become more transparent. There isn’t usually light behind the wet thing, so it appears darker. If you hold a piece of paper with a wet spot up to the light, the wet spot will look more see through.
It’s the same for concrete too – it’s mostly made of sand and other minerals that have transparent crystals. Water helps light get into the crystals instead of being reflected away.
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