[Igor (OP)]

Water is very reflective.
Why is it then that applying water to cloth makes the cloth reflect less light, (appear darker) ?

[I have a couple of ideas, but am interested in hearing the views of The Naked Scientists first]

[lightarrow]

Water is very reflective.
Why is it then that applying water to cloth makes the cloth reflect less light, (appear darker) ?
[I have a couple of ideas, but am interested in hearing the views of The Naked Scientists first]

1. Water lowers the refractive index rate between air and cloth (or other material, for example a wall). The reflection coefficient depends on this rate.
For normal reflection (the incident beam is at 0° from the interface axis, that is, at 90° from that surfacewe) we have:
I/I₀ = [(n-1)/(n+1)]²
where:
I₀ is the intensity of the incident beam
I is the intensity of the reflected beam
n is the rate between the greater and the smaller refractive indexes of the two media.
For example, air/glass: n ≈ 1.5 (depends on the type of glass)
So I/I₀ ≈ 0.04. It means that 4% only of the incident light beam is reflected by the (first!) glass surface.
air/water: n ≈ 1.33 --> I/I₀ ≈ 0.02 --> 2%
water/glass: n ≈ 1.16 --> I/I₀ ≈ 0.0055 --> ≈ 0.5%

2. Water absorbs light a little (in the red-infrared).

3. Water enters in the tiny interstices (I hope this term is correct) of the material, so light entering there and which is reflected many times before coming out, is forced to go through the water, which absorbs it a little at every reflection; so when light comes out has already a lower intensity.

[eric l]

Wet cloth is darker when you take only reflected light in consideration, not with transmitted light.  But indeed, raindrops are seen as dark(er) sports on cloth.
Part of the reason is actually that damp cloth transmits more light - as you see in those "wet tee-shirt contests" - which leaves less light to reflect.

[Igor (OP)]

3. Water enters in the tiny interstices (I hope this term is correct) of the material, so light entering there and which is reflected many times before coming out, is forced to go through the water, which absorbs it a little at every reflection; so when light comes out has already a lower intensity.

Like light in a fibre optic cable, Lightarrow ?

[lightarrow]

Wet cloth is darker when you take only reflected light in consideration, not with transmitted light.  But indeed, raindrops are seen as dark(er) sports on cloth.
Part of the reason is actually that damp cloth transmits more light - as you see in those "wet tee-shirt contests" - which leaves less light to reflect.

Did you read my post?

[lightarrow]

3. Water enters in the tiny interstices (I hope this term is correct) of the material, so light entering there and which is reflected many times before coming out, is forced to go through the water, which absorbs it a little at every reflection; so when light comes out has already a lower intensity.

Like light in a fibre optic cable, Lightarrow ?

Yes, something like it.

[eric l]

Wet cloth is darker when you take only reflected light in consideration, not with transmitted light.  But indeed, raindrops are seen as dark(er) sports on cloth.
Part of the reason is actually that damp cloth transmits more light - as you see in those "wet tee-shirt contests" - which leaves less light to reflect.

Did you read my post?

We must have been writing at the same time, I did not read your post before I posted mine (too bussy spell checking probably).