A glass windowpane with a thin film of water on it reflects less than when it's perfectly dry. Why?

Question

It's a question about interference in thin films

Answer 1

You're getting a double reflection, from the air-water interface and from the water-glass interface. The thickness of the water film is such that the additional path length traveled by the second reflection causes a phase reversal relative to the first, and thus destructive interference. Path length difference is an odd-integer multiple of a half-wavelength. Since, in this case, reflection itself causes a single phase reversal of each reflection, this means the water film is an odd-integer number of quarter-wavelengths in thickness.

If the film were not water but a material of higher refractive index than glass, the relative phasing would be different. Reflection phase reversal only occurs at a transition from lower to higher R.I. (both reflections in the water-film case, only one in this case); thus the first reflection would be phase-reversed but the second would not. For destructive interference the path lengths would have to differ by an integer number of full-wavelengths, so the film would have to be an integer number of half-wavelengths in thickness.

The ref. and its following page explains this well.