When the rain wets our suit, or we have just taken the wet laundry out of the washing machine, the fabric is darker than when it dries . This effect has to do with the refraction of light . Between the fibers of the tissues there are usually spaces, so that if a beam of light falls, they reflect it for the most part. If the fabric is wet, the interspaces fill with water, which means that a large part of the light rays are not reflected but rather refracted, that is, they are deflected at an angle other than the angle of incidence. In this way, wet fabrics reflect less light than dry fabrics , and therefore appear to be darker.
Something similar happens to wet sand, darker than dry sand.
