Wave interference
Two sources (A and B) oscillates in phase.
The distance between the sources is b (can be adjusted by the slider).
The wavelength of the two waves is lambda (can also be adjusted by a slider).
Wave from source A is red and travels to the right.
Wave from source B is blue and travels to the left.
The total wave is black.
Note the position of the nodal points of the total (black) wave. I have marked two of them, P and Q. There is a possiblity to have more nodal points than this.
|AP - BP| is always an odd number of half wave lengths
Inspect how many different nodal points you get when lambda is chaged.