Answer to the Question 11/96
The question was:Why wet spots are dark?
No correct answers have been received.
There are two explanations for the darkness of wet surfaces (see D.K. Lynch and W. Livingston, Color and Light in Nature, Cambridge U. Press, 1995):
1. If a thin layer of water overlays a non-porous substance then the light which in the absence of water was reflected from the surface of the substance to the eye of the observer (i.e. was not absorbed by the surface) reaches the the water-air interface and part of it is again reflected towards the surface, where it can be absorbed again.
2. If the substance is porous then part of the light is reflected from the surface (and has the typical color of the material) while another part of light is repeatedly scattered from the irregular features of the surface (some of which are of the size of the wavelength) and eventually reaches the eye of the observer. The latter part is essentially "white". This white light "dilutes" the "true color" of the substance. When the surface is covered by a thin layer of water the scattering is reduced, because now the light does not go from air (with refraction coefficient n=1) to material (with large n), but first goes to the water (with n=1.3) and only then to the material. The reduced contrast decreases the scattering. Thus reflected "true color" is no longer "diluted" by the white light. The substance looks darker and its color is "juicier".
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