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Optics - Focal length question

  1. Oct 29, 2011 #1
    1. The problem statement, all variables and given/known data

    For a lens with a focal length f, find the smallest distance possible between the object and its real image.

    2. Relevant equations

    1/s + 1/s' = 1/f

    3. The attempt at a solution

    I tried plugging in different numbers for the solution, but I'm not sure how to reach the solution..

    The answer is s + s' = 4f
  2. jcsd
  3. Oct 29, 2011 #2


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    What is the distance D between the object and image in terms of s and s'? Find the expression D(s). Differentiate D with respect to s and solve the equation D'(s)=0.
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