An experiment for the intermediate undergraduate optics laboratory is described which demonstrates the effects of the polarization of light on a double-slit Fraunhofer diffraction pattern. The experiment serves to illustrate in a single study the phenomena of diffraction, interference, coherence, some aspects of apodization, the Fresnel-Arago law, as well as some of the interrelations between these concepts. © 1979, American Association of Physics Teachers. All rights reserved.