interference physics

同相會加強振幅，反相會扺消振幅，光波聲波類同

If the two sources are in antiphase, places where there are constructive interference now become destructive interference, and places where there are destructive interference become constructive interference.

The reason is apparent. If the two sources are in phase, the condition for constructive interference is: path difference = n x wavelength (where n is an interger). But when the waves from the source are in antiphase, a shift of whole number of wavelength will be in antiphase, thus producing destructive interference.

Constructive Interference vs Destructive Interference