A stupid question. Why does water make cloth darker when it touches the cloth?

It's a bloody good question, not a stupid one. I'm not 100% sure that this answer is correct, but here goes. A large single crystal of sucrose, salt, a piece of glass or any other transparent material is, well, transparent; you can see through it. Grind it up into powder and it's opaque and white. That's because the powder contains lots of interfaces between the transparent material and another transparent material, air, with a different refractive index. Every time light passes between two materials with different refractive indices, some is reflected. If it's a powder and the surfaces are oriented at random, the light is scattered so you can't see through it. Water and cloth have refractive indices which are closer than air and cloth, so the effect is less. Similarly fat or water and paper fibres, so when you rub butter into opaque white paper, or wet it, it becomes translucent.

No... not stupid at all. These were the type of inquisitive minds that led to such discoveries as the Law of Gravitation to Raman effect to the transistor.

Any way you will notice that when the cloth is wet the fibres lose their stiffness and therefore their ability to reflect light the same way as the dry cloth. Hence the difference in the shades of brightness.

Emmm...

I think it's 'cause the cloth is soaking up the water.

And the water will be making the cloth go darker;

where the cloth has soaked up the water,

Hope that helped!

:D

I suspect that the lighter color is from light refracting off the curved strands. Water changes the relative refractive index because it is different that air. Just a guess but it sounds plausible.