Light from a distant source, consisting of red and green waves, is incident in a parallel beam perpendicularly to one of the refractive faces of a prism with a small refraction angle a. A lens with focal length F is installed behind the prism so that its optical axis coincides in direction with the light beam incident on the prism. It turned out that the distance between the images of the source in the focal plane of the lens in red and green light is equal to l. Determine the difference in the refractive index of the prism material for red and green light. To simplify calculations, the angles of deviation of rays from the optical axis of the lens are considered small, so that the sines and tangents of these angles can be approximately replaced by the angles themselves.

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