Probability of Absorption by a Wetted SurfaceThe parameter a in Eq. (1) is the fraction of the lightincident on the surface which is absorbed. (The aboverefers to a single interaction: the total probability ofabsorption, allowing for reflections at the liquid-airinterface, is A.) Angstrom takes a to have the samevalue for the wet as for the dry surface. We expect a,(the value when wet), to be greater than ad (the valuewhen dry), since the absorbing medium will normallyhave the real part of its refractive index greater thanunity. Since reflection is caused by wave vector mismatch,and since wave vectors are determined by refractiveindices, covering the surface with a layer ofliquid (with nj > 1) results in less reflection.The value of ad is in principle determined by thecomplex refractive index nr + ins of the material, andby the roughness of the surface (which influences theaverage angle of incidence on its randomly orientedfacets, and the probability of multiple interactions, asin a crevice). The value of a, is in addition a functionof nj, the refractive index of the liquid film covering it.For the purpose of comparing the albedos 1 - ad and 1- A of the dry and wetted surfaces, we estimate a, interms of ad, nl, and nr as follows.For small absorption (ni << nr), ad 1 - R(n) whereR(n) is the average reflectance of an isotropically illuminatedsurface, defined in Eq. (7). The assumptionmade here is that the angle of incidence on facets of therough surface (for, say, normal illumination) has thesame distribution as would be obtained for a planesurface illuminated isotropically. Similarly, a, - 1 -(nr/nl) when ni << n,. Thus when the absorption issmall
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