Refractive index of a material is the ratio between speed of light in vacuum and speed of light in that material. Light tends to bounce back when encountered with a sharp change in refractive index. Being wet means that there's a water film covering the material, mediating the change in refractive index, resulting in reduced reflection.
Apart from index mediation, the water film does something else. For rough/fibrous surfaces, the reflection will be diffuse, i.e. visible from all directions. When a water film is present, the surface becomes smooth, and the reflection will be specular, and only visible in one direction. So in most directions, the material will appear darker.
Conductors are a completely different beast. The reflection off of metals are not solely dictated by the refractive index.
> When a water film is present, the surface becomes smooth, and the reflection will be specular, and only visible in one direction. So in most directions, the material will appear darker.
Yes, that's precisely the part I was addressing in my last paragraph. If it's specular reflection, then in "most directions" it will appear darker, as you say, but in one direction it should appear brighter, even shiny. But I've never seen a damp rag be shiny in any direction.
(...and it shouldn't be that hard to see, if that effect is really true. With any other shiny object (polished car, CD, balloon) you see the specular reflection frequently.)
> But I've never seen a damp rag be shiny in any direction.
The surface of a cloth is still much less smooth than a polished car, cd, etc, so is it possible that the specular reflection happens, but at a contrast that is too low for our eyes to detect, or in enough disjoint sections that we can't perceive it as a single effect?
There are plenty of phenomena that fall out of the range of our unaided perception.
We would, however, probably observe the specular reflection of instead of water we used a thicker transparent liquid, like clear glue.
Apart from index mediation, the water film does something else. For rough/fibrous surfaces, the reflection will be diffuse, i.e. visible from all directions. When a water film is present, the surface becomes smooth, and the reflection will be specular, and only visible in one direction. So in most directions, the material will appear darker.
Conductors are a completely different beast. The reflection off of metals are not solely dictated by the refractive index.