I spent about 10 minutes working with this item. It is a simple and surprisingly effective qualitative demonstration of the interference of waves emanating from two slits. The applet is simple to operate, and generally straight-forward. There are some undocumented features: For example, the user can click and drag the slits closer together or further apart (however, the change is not reflected in the "distance" box). Also, clicking the right mouse button toggles the effect of the left mouse button. The applet is visually effective, with the regions of maximum interference clearly visible. Unfortunately, these regions appear white, the opposite from the black representing matching phase points (e.g., peak values). As is, the applet is a simple way to illustrate qualitative effects of constructive interference, although the explanation of a teacher needs to accompany its use. Its usefulness would be much improved by gray-scale intensity representation and/or the inclusion of some simple tools or information that would allow quantitative analysis.