This applet shown the diffraction/interference for multiplt slits.
Drag the [b]N[/b] slider bar to change the number of slits from 1-20. (1 for single slit)
Other slider bars at the bottom allow you to change the [b]Slit width[/b], [b]Slit separation[/b] and [b]Wavelength[/b] of the light source.
Slider bar on the top change the spatial resolution in the horizontal direction.
Slider bar at the right change the ntensity of the light source.
uncheck the checkbox to remove intensity plot (only display spectrum on the screen).

VIEW or DOWNLOAD ejs xml source
then click to translate strings for selected locale.

Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop.