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Building a telescope

  1. Dec 3, 2009 #1
    1. The problem statement, all variables and given/known data
    I need to build a telescope from two magnifying lens, one is 2x the other is 5x. First I need to give the total magnification possible, state which lens to use as the objective and finally to state the length of the telescope.

    2. Relevant equations
    l Ml = Fo/Fe total magnification is lM1l x lM2l Length = Focal length of objective + focal length of eyepiece

    3. The attempt at a solution The total possible magnification is 10X. I use the 2X for the objective because you need a long focal length and smaller magnification. I am stuck on how to get the length of the scope. I know the absolute value of M and can say that Fobjective is equal to lMl times F of eyepiece. But I don't have either focal length as far as I can tell and so I have no way to determine the length. I have the answer, so I know it should be 55 cm but why????? Any help would be much appreciated. Frostking
  2. jcsd
  3. Dec 3, 2009 #2


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    Homework Helper

    There's a formula that relates the magnification of a magnifying glass with its focal length. Do you know what it is?
  4. Dec 4, 2009 #3
    Yes, silly me it is M = 25/F On exams we are only allowed to use formulas on our formula sheet or that can be derived from the formula sheet. This formula is NOT provided. I guess I will just have to memorize this and hope that it is allowed to be used on the final. Thanks and sorry for wasting your time. Frostking
  5. Dec 4, 2009 #4
    This formula is valid for magnifying glass. You don't need it for telescope.
    For a telescope the magnification is given by the ratio of the two focal lengths.
  6. Dec 4, 2009 #5
    Thanks so much. I will remember this and use it! Frostking
  7. Dec 4, 2009 #6


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    You don't need the formula for a telescope, but the specifications of the magnifying glasses are given. I can't see how you would find the ratio of the focal lengths without using these specifications.
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