A Study of Brownian Motion Using Light Scattering

From the spectrum of light scattered by a suspension of particles in a fluid one can obtain quantitative information about the motion of the particles, including an accurate determination of their diffusion constant. If the incident light source is a laser, and the scattered light falls on the photosurface of a photomultiplier tube, then by measuring the spectrum of the photocurrent one obtains the spectrum of the intensity fluctuations of the scattered light. The intensity of the scattered light is determined by the instantaneous superposition of the phases of the waves scattered from each of the diffusing particles and the intensity fluctuates because the particles move. For particles of known diameter one can predict the spectral shape and width from the diffusion equation. We present a calculation of the spectrum of the field and the spectrum of the intensity of the scattered light and an advanced laboratory experiment and lecture demonstration by which the intensity spectrum can be studied.