Well wonder no more.
You might have observed that whenever a surface gets wet it becomes darker. Ever wondered why this might occur. Let's take the above image as an example. By seeing the image we can easily identify which side of the surface is wet. Well, the main reason it gets darker is due to two phenomena known as diffuse scattering and Total Internal Reflection.
Diffuse scattering: It is the scattering that arises from any departure of the material structure from that of a perfectly regular lattice. One can think of it as the signal that arises from disordered structures, and it appears in experimental data as scattering spread over a wide q-range (diffuse). Diffuse scattering is generally difficult to quantify, because of the wide variety of effects that contribute to it.
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Total Internal Reflection(TIR): Total internal reflection, in physics, complete reflection of a ray of light within a medium such as water or glass from the surrounding surfaces back into the medium. The phenomenon occurs if the angle of incidence is greater than a certain limiting angle, called the critical angle. In general, total internal reflection takes place at the boundary between two transparent media when a ray of light in a medium of higher index of refraction approaches the other medium at an angle of incidence greater than the critical angle. For a water-air surface the critical angle is 48.5°.
Because of refraction depend on wavelength, the critical angle (and hence the angle of total internal reflection) will vary slightly with wavelength and, therefore, with color. At all angles less than the critical angle, both refraction and reflection occur in varying proportions.
Detailed Reason:
You may have also observed that if the surface is smooth then we can't distinguish between wet and dry surfaces. This is because the surface smooth. And it reflects specularly regardless of the surface being covered with fluid. A ray of light reflecting at a smooth surface will not be totally reflected at the liquid-air interface. Now that we know that the surface needs to be rough. When the surface gets wet, the layer that is attached to it acts as the extra medium between the surface, air and our eye. This extra medium is responsible for the darkening of the shade of the color. Well, this is the simplest explanation of it. And there's more to it. Refer to the following paper for further information.
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