【单选题】
如图所示的薄膜上薄膜的折射率为n2,两边是空气折射率为n1(n1< n2),一束垂直入射的光经前后两个表面反射后的反射光相干的波程差是:()<br /> <img src="https://nimg.ppkao.com/2018-09/wangjing/2018093017134549011.jpg?sign=6ab83703e4b5941bb7c78d50f862213f&t=62d1b461" /><br /> <img src="https://nimg.ppkao.com/2018-09/wangjing/2018093017135495466.jpg?sign=579fdd56c9ebe1fa93d97a16c6f8f352&t=62d1b461" />
A、A
B、B
C、C
D、D