50. A thin film with index of refraction n1 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 λ within the film, maximum constructive interference between the incident beam and the reflected beam occurs for which of the following film thicknesses?
(A) 2λ
(B) λ
(C) λ/2
(D) λ/4
