In the a museum display on early astronomy is displayed an antique telescope. A

Question

In the a museum display on early astronomy 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 cm 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 (fe) of the eyepiece?

Explanation / Answer

M = fo / fe = 20 fo = 20fe Now, Final image can be viewed by relaxed eye. => final image forms at infinity => first image forms at focus of eyepiece => focus of eyepiece and objective coincide. So, fo + fe = 60 21fe = 60 fe = 60/21 = 2.857 cm

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