Why Do Wet Clothes Appear Darker?

[Kimia]

When we pour some water on our clothes the wet parts look darker than the dry ones! what is the logical reason for that?

First of all I should say this may seem too easy but it should have a detailed explanation because it is a valid question that was given for a professional competition.
I think it is because of the refraction but if we suppose so and consider the waterdrops like a sphere then we will see that there will be some decomposition of light and we will have all the visible lights but because there isn't enough water drops and most of the lights reflacted are caught by the molecules of the cloth we can't see them. (the refraction is like the refraction which happens in the raindrops when creating a rainbow) (but I somehow feel like something is not right with this explanation)
and it may also relate to the light polarisation which I couldn't find an explanation for it.
and we can say that because the light will be stucked in a place which was significantly smaller that the place it was, the light is pressed and its frequency will increase and the wavelength will decrease (but in that case it should become closer to blue not black!)
the last reason maybe because the water will get the energy of the energy of the light so it becomes warmer but I couldn't explain it in details.
[and I have seen the related discussions but they were about the surface not clothes I think there is a little difference between the surfaces that can't absorb water (like a road) and the surfaces that can (like clothes)]

 

[voko]

Cloth has a rough surface, which makes it scatter a lot of light. When it gets wet, it becomes much smoother, thus scattering less light. This also explains while thin wet white cloth gets translucent.

 

[phinds]

It's not at all about refraction, it's about reflection. As voko said, smother surfaces reflect very differently than rough surfaces. I take a lot of pictures of the end grain of wood and when it's rough-cut, it's very light and when it is very fine sanded, it's much darker. The is because of the way the light is reflected.

To see what I mean, check out the pics at, for example,

http://www.hobbithouseinc.com/personal/woodpics/oak , black.htm

and go to the "end grain update" link given there for further discussion.

Wetting material has the same effect as fine-sanding it. A polished steel sheet, on the other hand, will show no difference in color when moistened because it already HAS a very smooth surface.

 

[Okefenokee]

The refractive index of water is about 1.3. The refractive index of cotton is around 1.5 (I just looked it up). That tells us that dry cotton has a reflectivity of about 4% in an air cotton boundary.

Reflectivity = |(n1-n2)/(n1+n2)|^2

A water air boundary has a reflectivity of 2%.

A water-cotton boundary would have about 0.5% reflection.

It looks like when you wet a fabric you allow more light to reach the dye molecules because there is less reflection. Another way to say it would be that water makes the cotton fibers less visible because their indices of refraction are not too far apart.

 

[Borek]

Scroll down the thread, and see "related discussions" thingy at the bottom of the page, for many more discussions.