Answer: The reason that a surface appears darker when wet has to do with two main factors:
The refractive index of water.
The scattering of light by water molecules.
Step-by-step explanation:
The refractive index of a material is a measure of how much light bends when it passes through the material. Water has a higher refractive index than air, which means that light bends more when it passes from air to water. This bending of light causes some of the light to be reflected back into the air, which makes the surface appear darker.
Water molecules are much smaller than air molecules, and they can scatter light in all directions. This scattering of light causes the surface of a wet object to appear more diffuse, or spread out, which also makes it appear darker.
In addition to these two factors, the color of the object also plays a role in how dark it appears when wet. Objects that are dark in color, to begin with, will appear even darker when wet because the water will absorb more of the light that hits the surface. Objects that are light in color, to begin with, will appear less dark when wet because the water will reflect more of the light that hits the surface.