I. INTRODUCTION
The resolution limit of an optical instrument is given by
the Rayleigh criterion. This limit can be easily demonstrated
to students in a large lecture hall. The images of two light
sources are observed as a function of instrument aperture.
Having passed the limit of resolution by using too small an
aperture, it nevertheless is possible to detect the presence of
two unresolved light sources by invoking an elegant technique known as interferometry. This technique is presently
being used to resolve close binary stars and to measure the
diameters of single stars, which also can be easily demonstrated.