TRANSMISSION OF LIGHT BY WATER DROPS 1 TO 5 μ IN DIAMETER

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Abstract
The size of the drops formed when water vapour condenses is related to the work performed against the surface tension. For the determination of the size by optical methods, Mie's solution of the electromagnetic equations for the propagation of light of wave-length λ in a medium containing small spherical drops of radius a indicates that as the ratio α = 2πa/λ increases from 0 to 20, the intensity of the light received in the prolongation of the incident beam passes alternately through maximum and minimum values. At distances from the drops greatly exceeding λ, the first maximum lies close to α = 2π, the second is near α = 8.6, and the subsequent peaks are less distinct and tend to repeat themselves at α = (m + 3/4)π. As a result of these fluctuations the light seen through a cloud of particles with diameters greater than about 1 μ is coloured. The theory accounts for the cycles in the changes of colour observed when the diameter increases, and enables a determination of the radius of growing drops. With increasing radius, the influence of the index of refraction m decreases; for m = ∞ the positions and values of the peaks differ only slightly from those obtained with water.

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