All kinds of surfaces, from fabrics to rocks.
Someone spoke on this once:
http://www.straightdope.com/columns/read/816/why-do-wet-things-look-darker-than-dry-things
Huh. It has nothing to do with what I thought, then.
I figured it was similar to scratches on a polished surface (scratches being dry and polished being wet), in that a wet surface is just smoother. Except that I have no idea why a smoother surface would be darker…
A smoother surface would be darker from almost all angles, because it’s mostly reflecting the light that hits it in one particular direction. If your eye happens to be in that direction, you’ll see a glare, but in any other direction, it’ll look dark. Incidentally, this is why the “seas” on the Moon look darker; because they’re smoother than the craters.
Okay since that’s pretty well answered, how come some colored fabrics seem to temporarily change color when ironed?
I have a bright warm-red cotton shirt. When passed over with a hot steam iron, the color shifts more magenta. The difference between the hot ironed fabric and the cooler unironed fabric is pretty clear. But when the ironing is done, the shirt is back to its original color.
Could it be an effect of the heat on a particular dye? The steam? It’s not darker, really, just somewhat less orange.
Check out this very simple explanation: Video.
Very cool. Not sure how that works for rocks, but very cool for fabrics.
I hesitate to refer to the Master’s explanations as anything less than complete, but must say that I think he leaves an important and easy part out.
You know how transparent objects are less easy to see if you hold them underwater? For example, a crystal clear plastic cup is easy to see when it’s sitting on a table, but if you immerse it in an acquarium, it gets less visible? This is because we see the surfaces of transparent materials because they reflect some light. Likewise you can see your reflection in a window, more or less easily depending on what’s on the far side. The amount of light reflected by a transparent surface is a function of how much the index of refraction changes at the surface. The index of refraction of air is practically 1, and the index of refraction of water and some plastics are around 1.3 or so. A fair amount of light reflects when the index changes between 1 and 1.3, but not much does if it goes from, say, 1.323 to 1.341.
Part of the light we see looking at paper or rock or cloth is getting scattered by roughness at the surface, which on a microscopic scale is caused in part by reflections at the surfaces of somewhat transparent materials (and on a microscopic scale more things are transparent). Having water fill the void spaces in paper or cloth or rock (even if only slightly porous) makes all those complicated surfaces less effective at scattering light, so we see less light (it’s darker), and what we also see is more nearly limited to what the substance itself reflects (so the colors are fuller).
Even if no light is reflected from the interface, you’d still “see” the object by the distortions on whatever’s behind it. Again, though, those distortions depend on the difference in index.