机械波或者光波在介质的分界面处反射可能会发生半波损失的现象.半波损失与能量守恒都体现在波函数的求解结果中.本文以机械波为例，从幺正变换的角度理解半波损失发生的条件及其与能量守恒之间的必然关系.

For both mechanical wave and optical wave，the half wave loss might happen in the reflected wave at the interface of different media. The half wave loss and energy conservation can be inferred from the solution of wave functions. In this paper，we take the mechanical wave as an example to give the insights of the essential conditions of the half wave loss and its necessary connection with energy conservation from the perspective of unitary transformation.