Why do porous objects appear darker when wet?

December 12, 2003 4:00 PM

Why do porous objects (wood, textile, hair, etc) appear darker when wet then when dry? A googling gave me this ("surface reflection and refraction"), but that doesn't sound entirely reasonable, as it merely talks of light being scattered, but the light in question is already diffuse. This also gives some suggestions, but nothing conclusive. Any chem/physics MeFiers know this?
posted by fvw to Science & Nature (7 answers total)

See Here
posted by willnot at 4:05 PM on December 12, 2003

Response by poster: I don't buy it.

(1) Talk-Show-on-Commercial-Radio version: Because when something is wet, light bounces around inside it more (as opposed to merely bouncing off the surface) before being reflected back to the eye. The more the light bounces, the more of it gets absorbed, the less reaches the eye, and the darker the object appears.
Firstly, slightly damp fabric looks darker too, but no water surface is present due to water molecules not being close enough together (IIRC, correct me if I'm wong). Secondly, a fairly small fraction of diffuse light gets bounced off a water surface, which puts a fairly major limit on the degree to which reflection can be reduced by wetness.

(1) Talk-Show-on-Commercial-Radio version: Because when something is wet, light bounces around inside it more (as opposed to merely bouncing off the surface) before being reflected back to the eye. The more the light bounces, the more of it gets absorbed, the less reaches the eye, and the darker the object appears.
This sounds pretty much like the first link in the FPP. I don't understand this theory, as the light is already diffuse and so scattering shouldn't matter.

(3) the Ph.D.-thesis version, which comes complete with wavelengths, angstroms, and electron shells, but invariably the host's eyes start to glaze over and I find myself swiftly segueing into the latest on Rocky and Bullwinkle. Being the world's smartest human is all very well, but even I know when to quit.
Anybody got any links to someone who can tell this bit? I'm not afraid to get my hands dirty (metaphorically).
posted by fvw at 4:18 PM on December 12, 2003

It seems pretty basic to me: porous objects can hold water. Water can absorb light, lots of light. Absorbed light makes something darker.
posted by mathowie at 5:21 PM on December 12, 2003

I'm no expert, but perhaps it's due to the index of refraction of light (the same thing that makes you have to aim low when trying to gig a fish).

And FVW, I don't think it's a matter of porosity, but rather surface roughness. A rough stone will look darker when wet, too.
posted by notsnot at 5:47 PM on December 12, 2003

Response by poster: mathowie: Water doesn't absorb lots of light, it lets nearly all of it through, unless you have a very big mass of it. Remember, a glass of water doesn't look black.

notsnot: Refraction just changes the direction of the light, it shouldn't change the absorption (unless it somehow only directed the light into the object and not out). And as said, I have my doubts about any theory that uses refraction, I don't think there's enough water to form a surface in damp objects.

Regarding porousity/roughness: good point.
posted by fvw at 5:55 PM on December 12, 2003

This is a good question and you're right - the explanations linked abov dont make much sense. The explanation that makes the most sense to me is this one (see last question) - that basically water has an index of refraction closer to more materials than air does and thus leads light deeper into a surface where it has a greater chance of being absorbed rather than reflected.
posted by vacapinta at 6:44 PM on December 12, 2003 [1 favorite]

Response by poster: Vacapinta: That does make sense yes, and also checks out with gas+water emulsions appearing lighter. Thanks!

I don't think I even need mention anymore that I love AskMeFi
posted by fvw at 7:20 AM on December 13, 2003