We all have the almost instinctive ability to tell the difference between a wet and dry surface. Knowing that dark spots on clothes, blackboards, sand, roads and many other things usually indicate these to be wet spots. On top of that it also makes some materials more translucent, just think of a wet shirt.
The question that arises from all these observations is; “How can this be?”

Similar to the optical effects described in “why is snow white“, this also has a lot to do with diffuse reflections, as well as refractive indexes which play an important role.

Rays of light at verious angles

So what are refractive indexes? This is a number which describes how light will propagate when it encounters a different medium.
So if we look at water, which has a higher refractive index than air. We will see that light bends when entering or leaving the water. This is what causes objects which are half submerged in water to look distorted.

Next to this, when the angle of the light is increased, more light will get reflected rather than passing through. This and the refractive index are both very nicely illustrated in the picture above.

Now to return to wet surfaces, for example a piece of cloth, which like all other materials described above has a rough surface. This causes light that falls onto the surface of the cloth to scatter in a random way (diffuse reflection).
However this all changes when the cloth gets wet, as the thin layer of water drastically changes the material.

Firstly, the wet surface lessens scattering, forcing more light into the cloth.
Secondly, the water fills up the holes between the cloth fibers and causes even less reflection and scattering between the particles. It leads the light even deeper, causing more absorption and makes it easier for the light to reach the other side.
Finally, some of the light that is reflected out of the material by the cloth, is then reflected back into the cloth by the layer of water. (As can be seen in the second picture.)

A wet cloth showing a darker wet spot

All these processes added up result in less light coming back from a wet piece of cloth when compared to a dry piece, making it look darker.
When looking on the other side however, we can see that the wet spot looks lighter than the rest. This is because of the same process, but now the light source is on the back side of the paper, letting more light through the wet spot.
This can be seen in the picture above, where a wet piece of toilet paper is held in front of a mirror, thus showing both sides of the sheet.

Any rough surface can show this phenomenon; wood, concrete, cloth, asphalt, sand, dirt, etc. A smooth surface like marble or polished metals however, does not as there is no way for the light to get deeper into the surface.

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