I have noticed that wetting something almost always changes its colour. For example sprinkling water onto a t-shirt creates darker spots on the fabric; but this effect is not limited to fabric, almost everything once wet appears darker. Why is that?
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Maybe almost everything porous. But wet metal and wet plastic don't usually look any darker. Maybe that's a clue... – Solomon Slow Jun 17 '21 at 15:52
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Wet sand, cloth and many other substances with a porous microstructure become darker when wet because they become less reflective, both because of a reduction of diffuse reflection and because light penetrates deeper into the material. This explanation was originally formulated by Ångström in 1925.
When light hits an uneven dry surface like a sand grain, it scatters in all directions (diffuse reflection) while when covered with a smooth water surface most light is reflected in particular direction (specular reflection) that makes it look dark when you are not in its path. Light that enters water also can experience total internal reflection when trying to re-enter air, getting trapped and progressing deeper into the structure where it is eventually absorbed. This is helped by the scattering angle becoming smaller due to the higher index of refraction of the water compared to the index of air.
Anders Sandberg
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