Written by W. Christian at Davidson College.
Enter values for a new source:
x position: y position:
amp[0..1]: phase (degs):

Enter a new wavelength:

Additional comments by J. Varriano:
You can clear the sources with the Clear Sources button. You can add any number of sources one at a time. To add a source, enter its coordinates (in the same units as the wavelength), the amplitude of its wave, and the phase angle of its wave. Then hit the Add Source button. You can change the wavelength of the sources by entering the new wavelength (in the same units as the position coordinates) and hitting the Set Wavelength button. Finally, you can choose to see the amplitude of the resultant wave or the intensity pattern. The intensity pattern is more straightforward to interpret. White regions are bright regions.
You can read the coordinate of any point on the applet screen by moving the arrow to that point and holding down on the mouse button.

If you place several sources close together along a short line of width w, you can start to see the diffraction pattern for a single slit of the same width w. This verifies that one can think of the slit as being comprised of many closely arranged point sources and that the interference pattern of the many spherical waves gives the diffraction pattern. In fact, this is what is done in deriving the expression for the intensity distribution of the diffraction pattern for a single slit. (An integration of an infinite number of spherical waves is performed!)