This module is comprised of four separate submodules: (a) Single Slit, (b) Double Slit, (c) Diffraction Grating, and (d) Rayleigh Resolution. In (a) a monochromatic plane wave is normally incident upon a single slit. The resulting irradiance pattern is displayed on an observation srceen and the irradiance function is plotted as a function of longitudinal and transverse position from the slit. The user can vary the wavelength of the incident light, the width of the slit, and the position of the observation screen. In (b) the same thing is done for two slits, and the user can also change the distance between the slits. In (c) the user sends light which contains discrete spectral lines onto a transmission diffraction grating. The user is allowed to vary the width if the slit, the separation of the slits, and the number of slits. The resulting irradiance pattern is displayed on a semicircular screen that displays the full 180 degree angular subtense. In (d) light from two distant monochromatic point sources is incident upon a thin lens, and the resulting irradiance pattern in the focal plane of the lens is displayed on an observation screen. The user can vary the angular separation of the two sources, the wavelength of the light from the sources, and the diameter and focal length of the lens.