Multiplying one of the waves by a phase of minus one interchanges its peaks and troughs and thus changes constructive interference, a bright spot, to destructive interference, a dark spot.

• 如果彼此干涉的两个波之一改变了本身正负号（即此波函数多乘了-1这个因子），则此波的波峰与波谷就会对调，而将建设性干涉之处（一个亮点）变成破坏性干涉（一个暗点）。