A beam of white light (a continuous spectrum from 400 to 700 nm, let us say) is incident a...

A beam of white light (a continuous spectrum from 400 to 700 nm, let us say) is incident at an angle of 45° on two parallel glass plates separated by an air film 0.001 cm thick. The reflected light is admitted into a prism spectroscope. How many dark “lines” are seen across the entire spectrum?

Figure 1