For which visible wavelengths (in vacuum) will the film appear bright due to constructive interference

Question

A film of oil lies on wet pavement. The refractive index of the oil exceeds that of the water. The film has the minimum nonzero thickness such that it appears dark due to destructive interference when viewed in red light (wavelength=640.0 nm in vacuum). Assuming that the visuble spectrumextends from 380 to 750nm , for which visible wavelengths (in vacuum) will the film appear bright due to constructive interference ?

Thank you

Answer 1

n(oil)>n(water)

lambda = 640.0 nm

look for thin film equations