= Adjacent reflected minima of an air-film-air layer
{title2=$2nt=(1/\lambda_1-1/\lambda_2)^{-1}$}
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.
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