Past exam of the physics A course of the University of Cambridge / 2010 / Paper 1 / Section A / 2

For the adjacent reflected minima of an air-film-air layer in thin-film interference, reflection at the front surface acquires a phase reversal, while reflection at the rear does not. Near normal incidence the reflected minima therefore obey $2nt=m\lambda$ for integral m. A single intermediate maximum means that the two stated minima are consecutive orders:Thus $2nt=3000\mathrm{nm}$, with orders6 and5, andThe intermediate reflected maximum is at $\lambda =3000/5.5\simeq 545\mathrm{nm}$. This assumes negligible dispersion of the supplied refractive index and an air environment on both sides. ^[ref]