@Jeruba Okay, fine
When light moves from air to water, some of the light reflects and some refracts. The reflected light “bounces” off the water, and the refracted light bends at the air/water boundary and passes through the water.
When light strikes any object, some of the light is reflected and some is refracted and transmitted through or absorbed by the object. The relative amounts of which depend on the material properties of the object, its index of refraction.
When a material gets wet and absorbs water, the material’s index of refraction is effectively changed, making it so that more light penetrates and less light is reflected.
The light that is reflected from an object is the light that we perceive. How light or dark an object appears depends on how much light that strikes an object reflects back to our eye. For an object whose material has an index of refraction close to that of air very little light is reflected. For an object whose material has an index of refraction different than air, most of the light that strikes it is reflected.
When an object gets wet and absorbs water, its index of refraction effectively moves closer to that of air. When light strikes a wet object, therefore, less light is reflected than when it is dry. A pair of wet pants, a wet sidewalk, and a wet beach, therefore, reflects less light, and therefore looks darker. Steel, glass or plastic doesn’t look darker when it is wet because it doesn’t absorb any water, and therefore the same amount of light is reflected whether it is dry or wet.