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When something gets wet, it usually appears darker. This can be observed with soil, sand, cloth, paper, concrete, bricks ...

What is the reason for this? How does water soaking into the material change its optical properties?

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A nice treatment for this problem can be found here. –  Philipp Oct 28 '14 at 15:34
    
1 Spectral Reflectance of Wetted Soils asdi.com/getmedia/eeeb7ad1-c964-44fd-9404-d4870657e738/… –  amirrez kf Jan 31 at 5:47
    
Pour water on mirror. It won't be dark. –  SS-3.1415926535897932384626433 Feb 3 at 3:52

5 Answers 5

up vote 43 down vote accepted

When you look at a surface like sand, bricks, etc, the light you are seeing is reflected by diffuse reflection.

With a flat surface like a mirror, light falling on the surface is reflected back at the same angle it hit the surface (specular reflection) and you see a mirror image of the light falling on the surface. However a material like sand is basically lots of small grains of glass, and light is reflected at all the surfaces of the grains. The result is that the light falling on the sand gets reflected back in effectively random directions and the reflected light just looks white.

The reflection comes from the refractive index mismatch at the boundary between between air (n = 1.004) and sand (n $\approx$ 1.54). Light is reflected from any refractive index change. So suppose you filled the spaces between the sand grains with a liquid of refractive index 1.54. If you did this there would no longer be a refractive index change when light crossed the boundary between the liquid and the oil, so no light would be reflected. The result would be that the sand/liquid would be transparent.

And this is the reason behind the darkening you see when you add water to sand. The refractive index of water (n = 1.33) is less than sand, so you still get some reflection. However the reflection from a water/sand boundary is a lot less than from an air/sand boundary because the refractive index change is less. The reason that sand gets darker when you add water to it is simply that there is a lot less light reflected.

The same applies to brick, cloth, etc. If you look at a lot of material close up you find they're actually transparent. For example cloth is made from cotton or man made fibres, and if you look at a single fibre under a microscope you'll find you can see through it. The reason the materials are opaque is purely down to reflection at the air/material boundaries.

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I've spotted physics.stackexchange.com/q/30365 and this is effectively the same question. You might also want to look at the answers to that question. –  John Rennie Jun 19 '12 at 14:37
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The answer sounds plausible, however there is one thing which is still unclear to me: why when the wet surfaces freezes it turns bright again (often even brighter than before)? Refraction index of ice is more or less the same as for water. Perhaps the the ice not filling the space in between the grains as liquid water did? –  Suma Jun 19 '12 at 17:41
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I haven't done the experiment, but freezing may form a fine film of ice crystals on the top surface, and you'd then get diffuse scattering from the ice crystals. Maybe I'll try putting a water sand mixture in my freezer to see what happens ... –  John Rennie Jun 19 '12 at 17:46
    
I'm not convinced by this. There will be less reflected light, but it is a factor of two effect. Is that enough? Maximum transmission (minimum reflection) is achieved if the liquid has $n=\sqrt{1.54}$ for normal incidence and roughly halves the reflected fraction. –  Rob Jeffries Nov 20 '14 at 18:06

This image shows that when a surface is covered by water boundary, the light beams that face the surface, will absorb to it. so there would be less light beams in the area for arriving to our eyes. as a result, we see the material darker.

wet and dark

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I think this nicely adds to the the current top answer, which talks about air/sand and water/sand reflection, but misses the water/air transition description. Moreover, your answer is very easy to understand thanks to the picture. Perhaps a legend describing Ri, a and p would even improve it? –  Suma Jan 30 at 8:41
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The original image source seems to be victoria.ac.nz/scps/about/staff/pdf/darkerwhenwet.pdf, which also describe this in a detail, together with experimental verification of the theory. –  Suma Jan 30 at 8:46
    
@Suma +1 for the link –  Love Learning Feb 3 at 13:39

John's answer is the closest. Actually it does not matter of the optical index. Materials that has micro holes will reflect light randomly with only narrow boundary between the material and air. When there is water to fill up the holes, some of the light reflected into the material, so your eye will see less light reflected from the material.

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Hi Hong, I think the optical index does matter. Air (index=1) fills those holes too. Can you elaborate in your answer so it's more clear exactly what you mean? –  Brandon Enright Nov 21 '14 at 17:42

I was told by a physicist that it is because sand clumps together when it is wet, so there is less surface area within it that can reflect the light.

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This would not explain the phenomenon for concrete, asphalt ... –  Suma Jun 20 '12 at 20:29

The effect seems to be related to the fuzzyness of a surface.

Dry cloth is very fuzzy and therefore reflects light in more directions. If you wet it you bend small fibers on the surface towards it so that total reflection occurs in less directions.

It is comparable to glass, which also seems white when shattered into small enough pieces but only reflects into a small interval of angles when flat and intact. Another example is soap which is some color until you make a foam out of it.

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This explanation cannot be possibly true for the sand or soil - no fibers to bend on it. –  Suma Jun 19 '12 at 17:42
    
That's right. In sand or soil the liquid fills up the space between particles and thereby smoothens the surface. This effect also additionally takes place in fiber, of course. –  user9886 Jun 20 '12 at 8:37

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