FIG. 2.

A set of drawings depicting the Doppler effect on which the DLS measurement device relies in determining the size of the dispersed particles. The Doppler effect occurs when the source of waves is moving relative to the detector. Red shifts to lower frequencies result when the source moves away from the detector, while blue shifts to higher frequencies occur when the source moves towards the detector, independently of the waves in question: electromagnetic (galaxies in the image) or mechanical waves of pressure (train whistle). (Figure available in color online.)