ABSORPTION OF LIGHT BY SMALL DROPS OF WATER

When the extinction coefficient k in the attenuation formula I/I0 = (exp) — kz for a beam of radiations of initial intensity I0, and intensity I after travelling a distance z in water, changes from about 10−4, the value measured in the visible region, to 103, the value reached in some infrared absorption bands for water in bulk, the coefficient of extinction for water in very small drops, calculated according to Mie's theory, increases as long as the wavelength used is larger than the radius a of the particle. With larger drops, that is, when the wave-lengths are shorter than the radius, the coefficient for the extinction by water particles with strong absorption is smaller than that for perfectly transparent particles. However, the change does not exceed about 10% even where the absorption is strongest, and it is negligible when wavelengths smaller than one micron are considered. The main features of the coefficient of extinction per unit area of the drop remain unchanged by absorption; first there is a rapid increase with decreasing wave-lengths for particles of a given diameter until the ratio a/λ = 1/n (the reciprocal of the index of refraction) is reached, then follows a more gradual approach to a maximum, slightly less than 4πa2 as 2a/λ increases to unity, and finally, when λ is quite small, a decrease towards a constant value after a small number of fluctuations that reach their greatest amplitudes near the integer multiples of 2a/λ.