I made manually an app to explane the diffraction with .. order maximums.
I created several slides and put then next to each other.
See the diffraction.exe in the zip file.

This is very static. The quality isn't very good.
So maybe in HTML5 or Java there is more possible?

Basicly: sin theta = lambda/d

Changing the grid (d), wave length (lambda) and angle (theta) after the grid and see if the waves are in ore out of phase.
And with determined grid (d) and wave length (lambda) scanning the angles to see when the become in or out of phase.
Would this be possible?