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Simple magnifier problem

  1. Aug 4, 2007 #1
    1. The problem statement, all variables and given/known data

    A boy scout starts a fire by using a lens from his eyeglasses to focus sunlight on kindling 5.2 cm from the lens. The boy scout has a near point of 15.2 cm. When the lens is used as a simple magnifier, give values for the following conditions. (Hint: Assume a "normal" eye.)
    (a) What is the maximum magnification that can be achieved

    (b) What is the magnification when the eye is relaxed

    2. Relevant equations
    1/p + 1/q = 1/f
    Mmax= 1+ 25cm/f

    3. The attempt at a solution
    I used p=+5.2 and q= -(15.2)-((1/2)5.2)=-17.8
    then 1/5.2 - 1/17.8 +1/f
    f= 7.34
    so, Mmax= 1+ 25/7.34= 4.406 but that is wrong?!?
    can anyone help?
  2. jcsd
  3. Aug 5, 2007 #2
    You have to put everything in meters... you're using cm... check your units...
  4. Aug 5, 2007 #3

    Doc Al

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    Staff: Mentor

    That last equation assumes a near point of 25cm, but you are given a different near point to use.

    (No problem using cm everywhere, as long as you are consistent.)
  5. Aug 5, 2007 #4
    So i used the same equation except for Mmax I used 1+ 15.2/7.346= 3.068 but still the answere is not right?
  6. Aug 5, 2007 #5
    Please help soon as I do not have much time left to solve this problem.
  7. Aug 6, 2007 #6

    Doc Al

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    Staff: Mentor

    What's the focal length of the lens?

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