Collective Light Scattering: An Introduction

Abstract

Light scattering is one of the most common diagnostics of every day’s life. In addition to the object to be looked at, it requires only two devices, a light source, and a detector, which are both provided by the sun and the eye. The sun light that is scattered by solid body outer surfaces gives the first information we have of our environment. Light can also be scattered by other matter’s state than solid: “collective” light scattering refers to that kind of scattering that is obtained out of non-uniform transparent fluids. In that case, the scattered light can be thought of as an addition of electromagnetic waves that are scattered by each of the many individual particles constituting the transparent medium. The many atoms distribution (and motion into the gas volume) collectively shapes the scattered light intensity and frequency spectrum. Thus instead of a two-dimensional picture of an object outer surface, collective scattering provides a three-dimensional volume information. This is not so familiar: what is the three dimensional information provided by volume scattering, what does it mean, and how is it to be extracted and interpreted? These questions will be dealt with in two steps: by first investigating the scattering from a single particle. And second, by building the “collective scattering” as the result from a collection of independant atoms with a non-uniform density. The relation between the collective scattered light signal and fluid structure and motion will finally be analyzed.