If the applet does not popup, check your taskbar at the bottom
to see if it is open or download and install Java at
http://www.java.com/en/download/manual.jsp
 

This java applet generates Fresnel diffraction patterns, which are the shadows cast by small apertures when using a plane source of monochromatic light as a light source.

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.