system
Given the following post, respond as if you're commenting on Reddit:
human
Why do things appear darker and more colorful (e.g. stones) when they are wet?
gpt
The surface of many objects is rough on a microscopic scale, which is in large part responsible for the bright diffuse reflectance. When the microscopic roughness forming the surface is coated with water, the water fills in the tiny gaps which are otherwise filled with air. Because the difference in refractive index between the water and the stone is much less than the difference between the air and the stone, the wet rough interface will scatter less. There is also a new interface created (air/water). However, because the new boundary between the air and the water (covering the stone) is fairly smooth, it will not scatter light very much.