Interference: Optical path difference and phase difference etc. AHWatch
i) For thin films (say, oil on water), with a phase shift of radians at air-oil interface on reflection and a optical path difference from oil layer.
ii) For bloomed lens (coating on glass) with a phase shift of radians on reflection at BOTH air-coating and coating-lens interfaces and a OPD from coating.
Light gets a phase shift of radians on reflection when it moves from a medium of lower to higher refractive index, but not the other way around.
There are some diagrams and a more quantitative analysis here:
Light can also pick up a phase shift from being reflected if it moves from a medium of lower to higher refractive index. This occurs instantaneously at point of reflection and doesn't depend on OPD.
Hope that helps.