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Homework Help: Calculate the focal length of a planoconvex lens?

  1. Oct 27, 2016 #1
    1. The problem statement, all variables and given/known data

    Calculate the focal length of a planoconvex lens that produces a real image 2.5 times as big as the object if the object is 2.5 cm from the lens?

    2. Relevant equations

    The only one I can think of is 1/f = 1/v + 1/u

    3. The attempt at a solution

    My 'attempt' is 1/f = 1/62.5 + 1/25 = 0.056 but this just seems totally wrong.

    Had a whirl at it, but cannot produce an answer, any help would be really appreciated!
  2. jcsd
  3. Oct 27, 2016 #2


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    Staff: Mentor

    I'm not familiar with u and v in these equations. I think my class labels them differently. What do they represent here?
  4. Oct 28, 2016 #3


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    Homework Helper

    You have to invert the answer to get f :)
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