The app calculates a single layer reflection of wavelength and calculation of the phase. It shows the reflectance versus path difference graph, which depends on the refractive index and film of thickness.
The calculator program is following electromagnetic theory and electromagnetic theory and Frank J. Pedrotti, Leno S.Pedrotti, by Frank J.Pedrotti, Leno S.Pedrotti., [Introduction to Optics (2nd ed)], Prentice-Hall, Upper Saddle River Publishers, N.J. 391-396, (1993).