Two-dimensional wave overlay
Imagine two circular two-dimensional waves, generated respectively by a source F1 and F2, with equal amplitudes and frequencies, and in phase agreement.
Consider the schematicization of the interference caused as:
In the figure the left wave has ridges represented by black continuous lines and valleys by red dashed lines and the right wave has ridges represented by green continuous lines and valleys by blue dashed lines.
The filled circles represent points of constructive interference, ie where the amplitude of the waves is summed.
Blank circles represent destructive interference points, ie where amplitude is subtracted.