Light reflects from a thin film |
 |
|
The upper air-water surface in the diagram is a closed boundary; the phase change is 180°. The lower water-air surface is an open boundary. The phase change is zero. If the path difference between reflections 1 and 2 is one half wavelength, the two reflections will be in phase, and maximum light intensity will be reflected. If the path difference is a multiple of one half wavelength the two reflections will be out of phase and cancel. |