Consider a soap film where both sides of the film is air. So reflected light has a π phase change. Consider constructive interference, 2 d "nfilm" + 1/2λ = nλ , where "nfilm" is the refractive index of the film and d is the thickness the film. Due to the gravity, the film at the bottom is thicker. My teacher said that if the film is too thick, the above equation can be satisfied by more than one wavelength in the visible spectrum. So different colour combine to give a white colour. I don't understand what is the meaning of this statement. Is it mean that for different colour (λ), there will be a different order (n) to satisfy 2 d "nfilm" + 1/2λ ?? thank you!