Why do some materials appear darker when wet?

[cmb]

I say 'some materials' because it is not true for all. Usually, clothes appear darker when wetted, but I have one pair of trousers that actually goes ligher when water spashes on them, so it is not 'always'.

 

[Yuqing]

If you are very interested you can read this http://www.victoria.ac.nz/scps/staff/pdf/darkerwhenwet.pdf" which mathematically models the reflection of light for wet objects. The general idea is that when an object is wet, light is scattered and reflected more leading to an increased tendency for the reflecting object to absorb light instead.

 

[cmb]

OK, as far as it goes (thanks for the link). So why do some materials look lighter when wet?

 

[Yuqing]

Which materials in specific do you have in mind?

 

[cmb]

I don't know what my trousers are made of. It says its a synthetic/natural fibre mix. But are my trousers really the only material known that displays this property?

 

[Yuqing]

I'm guessing this is similar to when white T-Shirts and paper turn translucent when wet. Outside of this transparency I really cannot think of any clothing which becomes lighter when wet. (Although admittedly I haven't paid too close attention to wet clothes )

 

[phinds]

Some materials (I'm thinking of wood specifically) also get darker when polished as opposed to rough but I have no idea whether or not it's due to the same mechanism.

It MAY be closely related, because the mechanism in wood is that rough wood had a HUGELY larger surface area (due to little troughs and grooves) and thus reflects more light. Polishing reduces the surface area. Maybe wetting "smooths out" the surface area in a similar way.

If that's the case, it is possible that your special pants may be of a slick material that actually gains a surface roughness when moisture raises the fibers, thus it reflects more light.