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Homework Help: An Antique Telescope

  1. Mar 6, 2008 #1
    1. The problem statement, all variables and given/known data

    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 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?

    2. Relevant equations

    Length (L) = Fo+Fe
    Which Fo = Focal length of the objective lens
    Fe = Focal length of the eyepiece

    M = -Fo/Fe

    3. The attempt at a solution

    L = Fo+Fe
    60 = Fo+Fe
    Fe = 60-Fo

    M = -Fo/Fe
    20 = -(60-Fe) / Fe
    Which Fe would be -3.16

    But I got wrong..
    Please help me..

    thank you..
  2. jcsd
  3. Mar 6, 2008 #2


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    Hi xinlan! :smile:

    I don't know much about optics …

    Why have you put a minus there? :confused:
  4. Mar 6, 2008 #3


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    Make the angular magnification -20, that is the final image is inverted when viewed throught the eyepiece. Which would give you a positive focal length for the eyepiece as required for a convex lens.
    Last edited: Mar 6, 2008
  5. Mar 6, 2008 #4


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    But the question says "two converging lenses" - wouldn't that make both negative? :confused:
  6. Mar 6, 2008 #5
    Even though the M is -20, but the answer would still be the same..
    it just change the minus sign into positive sign..
    I put the positive focal length and I still got wrong
  7. Mar 7, 2008 #6
    I got it..
  8. Feb 2, 2010 #7
    How'd you find it?

    Here's How:

    20= (60-Fe)/Fe
    Fe = (60/21)

    Fe= 2.857
    Last edited: Feb 2, 2010
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