Extinction of light in dispersive media with high particle concentrations: applicability limits of the interference approximation

We investigated the limits of applicability of the interference approximation (ITA), which is often used to calculate light fields in dispersive media with high particle concentrations. We analyzed the conditions under which the use of the ITA leads to an error in calculation of the dispersive-medium extinction coefficient epsilon equal to 10%, 15%, and 25%. The analysis was carried out by comparing the values of epsilon calculated in the ITA with the results of the calculation of epsilon in a much more exact quasi-crystalline approximation (QCA). The validity of the QCA in estimating the error of the ITA is shown by comparing the values of epsilon calculated in the ITA and the QCA with experimental data [J. Opt. Sec. Am. 72, 1317 (1982)]. Our investigation shows that the ITA can be used only at relatively small values of the particle size parameter x. When the relative refractive index of the particles is n greater than or similar to 1.8, the applicability of the ITA is limited to the case in which the particle size parameter x less than or equal to x(max) approximate to 1.5. A decrease in n leads to an increase in x(max), and for nonabsorbing particles at n less than or similar to 1.3, x(max) is approximately proportional: 1/(n - 1). An increase in the imaginary part of the relative refractive index of the particles kappa leads to a decrease in x(max). The smaller the n, the greater the dependence of x(max),, on kappa. (C) 1999 Optical Society of America [S0740-3232(99)00505-0].