<image 1>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?
['2$\\lambda $', '$\\lambda $', '$\\lambda $/2', '$\\lambda $/4']
The film has a higher n compared to both sides, such as soap surrounded by air. 