Why do wet materials become darker?
INMACULADA PASCUAL VILLALOBOS answers:
When we look at a dry surface, a piece of fabric or even the sand, we perceive their colours. This is because these materials absorb incident light (electromagnetic radiation) selectively and reflect the range of light corresponding to the perceived colour. Dry materials reflect more light than wet materials, so we perceive more vivid colours when materials are dry.
If we wash clothes or soak a material in water, the light that reaches that material is again partly absorbed and partly reflected, but having water between the molecules of the material or in the free spaces between the fibers of the fabric, the phenomenon of refraction takes place. The fraction occurs on the separation surface between two media with different refractive indexes, in this case air and water. Water’s refractive index is greater than air’s, so when the light reaches a wet material refraction increases and reflection decreases. That is why we continue perceiving the same colour, but it seems darker because the amount of reflected light is lower due to refraction.
When the material dries, water evaporates and the refraction of light does not occur, so we can again see more intense colours.
Inmaculada Pascual Villalobos is a professor at the Department of Optics, Pharmacology, and Anatomy of the University of Alacant.