Retroreflectance from a dense distribution of spherical particles

TLDR

Backscattered intensity from a dense latex microsphere distribution is measured near the retroreflection direction. At volume densities above 1 %, a sharp retroreflection peak appears whose angular width is far smaller than λ/D and cannot be explained by Mie, double‑passage, or radiative‑transfer theory, the peak grows with particle size (2–4 λ) and density, and for D≫λ it overlays the Mie pattern, with width on the order of a wavelength divided by the mean free path.

Abstract

The backscattered intensity from a dense distribution of latex microspheres is measured near the retroreflection direction. It is shown that a sharp peak appears in the retroreflection direction when the volume density is above 1%. The angular width of this peak is much smaller than (wavelength)/(particle size) and cannot be explained by Mie theory, double-passage effects, or radiative-transfer theory. When the particle size D is less than the wavelength λw, a small peak appears at the retroreflection direction. When D is 2–4 times greater than λw, the peak becomes large as the density increases. When D is many times greater than λw, the sharp peak at the retroreflection direction is superimposed upon the Mie-scattering pattern. The angular width of the peak is of the order of (a wavelength)/(a mean free path).