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Homework Help: Two lenses: find Di through second lens

  1. Nov 26, 2016 #1


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    1. The problem statement, all variables and given/known data
    A 1.60 cm tall object is 20.0 cm to the left of a lens with a focal length of 10.0 cm . A second lens with a focal length of 36.0 cm is 44.0 cm to the right of the first lens.

    2. Relevant equations
    1/f = 1/di + 1/do , hi/ho = di/do

    3. The attempt at a solution

    calculate distance of image

    1/f = 1/di + 1/do
    1/10cm - 1/20cm = 1/di

    20cm = di

    with a image twice the length of the object, it must be a converging lens. its inverted and real.
    image of first lens becomes object of second lens.

    do = 20 +44 = 64cm

    f = 36cm

    1/36 - 1/64 = 1/di

    82.28cm = di

    But MP says its wrong....?

    The only thing I can think of is that the signs are wrong somewhere, but the image is real so it should be positive..... Any and all help is appreciated!
  2. jcsd
  3. Nov 26, 2016 #2
    Your first image is a real image to the right of the first lens so how far is it from the second lens?
  4. Nov 27, 2016 #3


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    yup that makes sense. Thanks for the help Cutter!
  5. Nov 27, 2016 #4
    You're welcome
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