Huygens principle

According to Huygens' principle, a wave can be interpreted as the sum of many individual waves. The individual waves can superimpose and increase or decrease in amplitude as a result of the superposition.

According to Huygens, an increase in amplitude occurs when the superimposed waves have no phase shift with respect to each other. If the amplitude is reduced, the waves have a phase shift against each other.

So not only the wave amplitude is decisive for the resulting waves, but also the phase position of the waves against each other. If two waves are identical in their amplitude but opposite in phase, the resulting wave will cancel.