Light reflects from both surfaces of a thin film

Light reflects from the closed boundary (above) and the open boundary (below).

There is a phase change of 180° at the upper (closed) boundary. 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. It can be shown that the inclusion of all subsequent partial reflections does not alter the result.