Dynamic light scattering

A HeNe laser beam incident on a sample of micron-sized spheres suspended in water scatters into a random pattern of spots of varying size, shape, and intensity. The pattern results from the coherent superposition of the outgoing waves scattered from the spheres. Because the spheres are in constant Brownian motion, the pattern randomly changes in time. A photodetector is placed in the pattern and the random fluctuations in the light intensity are measured and analyzed. The signal’s autocorrelation function and power spectrum are computed and used to verify the Brownian motion and determine the spheres’ diameter.