interesting suggestion!
i search the net and i found this

<iframe title="YouTube video player" class="youtube-player" type="text/html" width="480" height="390" src="" frameborder="0" allowFullScreen></iframe>

time = 25 sec seems to suggest diffraction did not occur for a large slit width as the wave plane just travel through the slit as if there is no obstacle.

anyway, your ejs applet above, can already simulate a large slit i think.

simulation are models, the are close approximation of the real physics
i tried but like cannot get it because it is very lagging. i think i need to restart my PC.
i manage to change from interpolated to grid seems to run faster now.
and select n=8 source, n=100 is very lag for my PC.
i think the ejs applet model is close enough to the real life setup.  ;D
not to worry then  :D