Why do the surface of objects always appear darker when they are wet?

I like that question.
I’m curious too.
The first thing that came to mind when reading your question were sweaty armpits.

Omg I don’t know the answer but my head just exploded!

When light strikes an object, some of it penetrates the object, and some of it is reflected and reaches your eye. When an object is wet, more light penetrates the object, so less light is reflected. As a result, less light reaches your eye and so the wet object looks darker

@teh_kvlt_liberal Wow, teh, neat to hear this coming from you. No offense. :P

I never even thought about this until now. I’m givin’ it a GQ. :)

@teh_kvlt_liberal, I’m not satisfied. Why is less light reflected off water when water itself is so reflective? That does not sound right to me. I’d like to see a more thorough explanation.

Are you sure it doesn’t have anything to do with the properties of the particular surface? Some surfaces are more porous or absorbent than others (for example, wood, dirt, paper), some (like metal) are smoother and some have more little gaps or depressions (like concrete), some are organic (leaves, fur), and some don’t change (glass). Doesn’t the behavior of water vary on different surfaces, and don’t the light effects vary as well?

@Jeruba Okay, fine
When light moves from air to water, some of the light reflects and some refracts. The reflected light “bounces” off the water, and the refracted light bends at the air/water boundary and passes through the water.
When light strikes any object, some of the light is reflected and some is refracted and transmitted through or absorbed by the object. The relative amounts of which depend on the material properties of the object, its index of refraction.
When a material gets wet and absorbs water, the material’s index of refraction is effectively changed, making it so that more light penetrates and less light is reflected.
The light that is reflected from an object is the light that we perceive. How light or dark an object appears depends on how much light that strikes an object reflects back to our eye. For an object whose material has an index of refraction close to that of air very little light is reflected. For an object whose material has an index of refraction different than air, most of the light that strikes it is reflected.

When an object gets wet and absorbs water, its index of refraction effectively moves closer to that of air. When light strikes a wet object, therefore, less light is reflected than when it is dry. A pair of wet pants, a wet sidewalk, and a wet beach, therefore, reflects less light, and therefore looks darker. Steel, glass or plastic doesn’t look darker when it is wet because it doesn’t absorb any water, and therefore the same amount of light is reflected whether it is dry or wet.

Bravo, @teh_kvlt_liberal! Thank you. I hardly ever turn down more explanation. (I know sometimes people would prefer less from me, but I like things to be complete, or at least as complete as they have to be to make sense.) So—the key to the answer is “index of refraction.”

@teh_kvlt_liberal You’re in-depth explanation made me get curious and pour a little water on my desk!
Great explanation! Bravo!