Question: A thin film with index of refraction n_1 separates two materials, each of which has an index of refraction less than nf. A monochromatic beam of light is incident normally on the film, as shown above. If the light has wavelength $\lambda $ within the film, maximum constructive interference between the incident beam and the reflected beam occurs for which of the following film thicknesses? A) 2$\lambda $  B) $\lambda $  C) $\lambda $/2  D) $\lambda $/4. Please choose an answer from [A, B, C, D].
Answer: D