Why wet hair is darker than dry hair?

This rule seems to work for paper and clothes too.

Why wet hair is darker than dry hair?

The water film facilitates trapping of the light within it via multiple reflections until it is absorbed. That is, since water is transparent, it acts as sort of an antireflection coating. Instead of mostly reflecting off the surface of the water, as it would for white paper, most of the light transmits through the water-air interface. When the light then reaches the paper, water's index of refraction reduces the amount reflected there. The light that *is* reflect, must then get back through the water-air interface and some of this is reflected.

This general technique, with optimized coating thicknesses and carefully chosen refraction indecies, is used to make antireflecting coatings on camera lenses and such.

Why wet hair is darker than dry hair?

Strangely enough, it is darker, I never thought of that.... but unfortunately I don't know why though.

Why wet hair is darker than dry hair?

The thin film of water absorbs light due to a phenomenon called destructive interference. So, whether you use oil or water, a thin film (at a certain thickness and multiples) causes the phases of light from the top and the bottom of the film to cancel out. At other thicknesses, some particular wavelengths get absorbed leading to different colors, esp. oil on water films.