system
Given the following post, respond as if you're commenting on Reddit:
human
Why do things get darker when wet?
gpt
**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.

**Edit**

Part 2 of the story

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.