ht ray's wavelength :  380 nm  to  760 nm  for visible light (white rays). μ_film = 1.33 thickness of film = d = 320 nm θ1 = 0°  = angle of incidence at the top layer of film μ_air = 1 θ2 = angle of REFRACTION into the soap film = angle of reflection at the film-air interface.    μ_air Sin θ1  = μ_film  Sin θ2          --- (1)   There is a phase shift of 180 DEG. when light rays are reflected at the top surface of the film, as μ_air < μ_film.  There is no phase shift when rays are reflected at the  film-to-air interface.  The OPD (optical path difference) is:         2 μ_film  d  Cos θ2  = (m - 1/2) λ   for constructive interference.    --(2)   2 μ_film  d  Cos θ2  = m λ    for destructive interference.    -- (3)                   here m = an integer.  As θ1 = 0°,  θ2 = 0°.      2 * 1.33 * 320 nm  = Optical path difference OPD                   = 661.12  nm   In equations  (2) and (3),  the RHS is λ/2, λ, 3λ/2 ...  We have the range of white light wavelengths mentioned above.  661.12 = λ for red color  satisfies equation  (3).  So red color is destroyed.  661.12 = 1.5 λ for  λ = 440.7 nm satisfies equation (4),   ie., violet color is constructively added.