On one of the shelves in your physics lab is displayed an antique telescope. A s

Question

On one of the shelves in your physics lab is displayed an antique telescope. A sign underneath the instrument says that the telescope has a magnification of 20 and consists of two converging lenses, the objective and the eyepiece, fixed at either end of a tube 60.0 long. Assuming that this telescope would allow an observer to view a lunar crater in focus with a completely relaxed eye, what is the focal length of the eyepiece? Note that to view the crater with a completely relaxed eye, the eyepiece must form its image at infinity.

Explanation / Answer

eyepiece focal length = objective focal length / magnification M = fo/fe and l = fo+fe Given M = 20, and l = 60. So, equation 1 is fo/fe = 20, which can be arranged as fo = 2fe. And equation 2 is fo+fe = 60 Solve for fe, and you'll get 2.86 cm.

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