Asked by: Carel Lucas, Perth, Australia
When light strikes dry hair, some of it is absorbed by the pigment in the hair and some reflects back to your eyes. Water is transparent and a thin film absorbs very little light by itself. But when light hits wet hair, some of the light reflecting off the surface of the hair strikes the inside surface of the water film at the right angle to be reflected or refracted back onto the hair again. This creates a second chance for the light to be absorbed, making the hair appear darker.