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Object distance and magnifiation

  1. Feb 4, 2009 #1
    1. The problem statement, all variables and given/known data

    A 6mm diameter microscope objective has focal length 9mm.

    What object distance is required to achieve a lateral magnification of -40?

    2. Relevant equations

    mobj=-L/fobj


    3. The attempt at a solution

    -40x=-L/9mm

    L=360mm

    Why isn't the answer 360mm? I think it might have something to do with the diameter of the lens, but I'm not completely sure. Can someone explain? Thanks!
     
  2. jcsd
  3. Feb 5, 2009 #2
    Anyone?
     
  4. Feb 5, 2009 #3

    Redbelly98

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  5. Feb 5, 2009 #4
    Well, the formula I included is basically an approximation derived from the Gaussian lens formula...at least, that's how my book put it.

    Basically, with a microscope, the object is very very close to the focal point, so s[tex]\approx[/tex]f. Similarly, the focal length of the objective is much less than the length of the microscope tube, so s'[tex]\approx[/tex]L

    Is it wrong to use these adapted formulas from the book?
     
  6. Feb 5, 2009 #5

    Redbelly98

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    Yes, the object distance will be close to f=9mm. So your answer of 360mm is way way off.
     
  7. Feb 5, 2009 #6
    Hmmm, after some thinking I got:

    [tex]1/f=1/s+1/s'[/tex]
    [tex]1/9mm=1/x+1/40x[/tex]
    [tex]x=9.225mm
    [/tex]

    And it was right! Thanks for the suggestion to use the Gaussian optics formulas :)
     
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