 Adjacent reflected minima of an air-film-air layer

With one reflection phase reversal and near-normal incidence, reflected minima satisfy $2nt=m\lambda$. Subtracting the reciprocal-wavelength conditions for consecutive minima gives the displayed thickness relation. A single maximum between two stated minima identifies the consecutive-order case. Wavelength-dependent refractive index or non-air surroundings can change the inference.