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Homework Help: Diverging and Converging Lenses

  1. Oct 3, 2014 #1
    1. The problem statement, all variables and given/known data
    A diverging lens (f = –13.0 cm) is located 18.0 cm to the left of a converging lens (f = 31.0 cm). A 3.30-cm-tall object stands to the left of the diverging lens, exactly at its focal point.
    Determine the distance of the final image relative to the converging lens

    2. Relevant equations
    1/f = 1/v + 1/u

    3. The attempt at a solution
    1/-13 = 1/13 - 1/v
    1/v = -2/13
    v= -6.5
    The substituting that v value into lens maker formula again for converging lens using f=31
    6.5+18 = 24.5cm meaning the image of first lens is the object for second lens which is 24.5cm away from the second lens.
    1/31 = 1/24.5 + 1/v
    This gives a v value which is negative which is wrong for a converging lens.
    What have I done wrong? How do I correctly solve the problem?

  2. jcsd
  3. Oct 3, 2014 #2


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    The image distance for a converging lens isn't always positive.
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