I modified the above applet (add more constraints) so that it will be closer to the real case.

About your request to generate something similar to the following image.

[img]http://ngsir.netfirms.com/applets/diffraction/diffraction1.png[/img]

Do you think the above image is a real wave? I am not so sure!

The intensity of diffracted wave should be much weaker than what has been shown in the above image.
And I do not think you will find any pattern (as found in the animation at http://ngsir.netfirms.com/applets/diffraction/2X/Diffraction.htm).

I will try to create one if you can show me a real experiment (video) which will generate pattern similar to the above image.  ;)