Color Effects in the Scattering of White Light by Micron and Submicron Spheres*

Abstract

The tristimulus color system is used to describe the colors of the light scattered by spheres with radii a = 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, and 1.2 μ for real refractive indices m = 1.31 and 1.46 and by spheres with radii a = 0.1, 0.2, 0.4μ for complex refractive index corresponding to a highly absorbing substance such as vanadium pentoxide. The quantities calculated are the dominant wavelength, purity, the visible intensity, and the intensity for unpolarized incident white light from a 3200°K source as well as for each of the two plane-polarized components.The angular variation of colors designated as higher-order Tyndall spectra (H.O.T.S.) occurs only for radii comparable to the wavelength. For small particles, the light is blue, corresponding to Rayleigh scattering, while for radii a>0.8 μ, the scattered light is uniformly yellow. At intermediate sizes 0.2 μ≤a≤0.8 μ the H.O.T.S. described by Sinclair and La Mer occur. However, there is no correlation between the appearance of the color bands and the intensity peaks of the scattered light. It is possible to have a polydisperse system in the intermediate size range exhibit H.O.T.S. while a perfectly monodisperse system of larger radius does not exhibit this phenomenon. A visual measure of the polarization ratio at two or three backward directions can serve to determine the particle size.