A soap film (index of refraction nf=1.33) is suspended in air. White light is incident from air onto the film at normal incidence.
(a) Identify whether a 180∘ phase change occurs for each of the first two reflections that contribute to the observed interference pattern. Justify your answer in terms of relative indices of refraction.
(b) Explain why a film thickness equal to one quarter of a particular wavelength inside the film produces destructive interference in the reflected light for that wavelength at normal incidence.
(c) When the same film is viewed at a large oblique angle, the dominant reflected color shifts toward shorter wavelengths. Describe qualitatively why increasing the angle of incidence in air causes this color shift, referring explicitly to changes in optical path difference and any phase factors.
(a) Identify and justify the phase change at each of the first two reflections.
(b) Explain the destructive-interference condition for a quarter-wavelength film at normal incidence.
(c) Describe qualitatively why the dominant reflected wavelength becomes shorter when the viewing angle is increased.