Question: Why does a wet spot on...

Question: Why does a wet spot on a shirt or other surface always appear darker than the surface itself?

Answer: The short answer is because the water reflects less light than the surface, making it appear darker. In a wet spot on a nonporous surface, for example, some of the light that passes through the water and reflects off the surface is internally reflected again by the air-water interface, allowing the light to be scattered and absorbed. In a porous substance, such as fabric, the water allows some of the light to pass farther into the material’s interior, where it is again scattered and absorbed.