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Author Topic: Why is damp cloth darker than dry cloth ?  (Read 3695 times)

Offline Igor

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Why is damp cloth darker than dry cloth ?
« on: 18/04/2007 16:22:47 »
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]


 

Offline lightarrow

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Why is damp cloth darker than dry cloth ?
« Reply #1 on: 18/04/2007 17:11:06 »
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.
 

Offline eric l

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Why is damp cloth darker than dry cloth ?
« Reply #2 on: 18/04/2007 17:13:43 »
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.
 

Offline Igor

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Why is damp cloth darker than dry cloth ?
« Reply #3 on: 18/04/2007 18:42:33 »
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 ?
 

Offline lightarrow

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Why is damp cloth darker than dry cloth ?
« Reply #4 on: 18/04/2007 21:32:46 »
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?
 

Offline lightarrow

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Why is damp cloth darker than dry cloth ?
« Reply #5 on: 18/04/2007 21:34:00 »
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.
 

Offline eric l

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Why is damp cloth darker than dry cloth ?
« Reply #6 on: 19/04/2007 15:20:57 »
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).
 

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Why is damp cloth darker than dry cloth ?
« Reply #6 on: 19/04/2007 15:20:57 »

 

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