Asterate

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.

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