While two (or more) electromagnetic waves of the same frequency, same direction of vibration, same phase or constant phase difference superpose in space, the amplitude of composite wave is vector sum of the amplitude of each wave.

• 由两个（或两个以上）频率、振动方向相同、相位相同或相位差恒定的电磁波在空间叠加时，合成波振幅为各个波的振幅的矢量和。