When you start up the applet, a circular aperture will be used. This is the pattern generated when a plane source of monochromatic light is shined on a small circular hole (about 5 mm across) in an otherwise opaque object. The red line is the outline of the geometrical shadow (the shadow which would be cast if there were no diffraction).
You can select a different aperture shape (such as a half plane, double slit, triple slit, rectangle, etc.) from the Aperture popup. Or you can change the size of the circle by dragging the red line with the mouse.
Version 1.1 posted 11/7/11
Zip archive of this applet.
Another Fresnel Diffraction Applet.
Another Slit Diffraction Applet.