Two converging lenses having focal lengths F1 and F2, respectively, are located on a common optical axis. At what distance between

Two converging lenses having focal lengths F1 and F2, respectively, are located on a common optical axis. At what distance between the lenses will a parallel beam of rays falling on the first lens leave the second also parallel?

For a parallel beam of light to travel behind the lenses, it must be emitted by a source in focus. A beam of light falling on the first lens will be collected at its focus. This luminous point should be the source for the second lens. Thus, the distance between the lenses should be equal to the sum of their focal lengths.
Answer: F1 + F2

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