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Optics, lens problem pleaase help

  1. Jan 15, 2008 #1
    Optics, lens problem pleaase help :)

    1. Two converging lenses, the first with focal length 40 cm and the second with focal length 33 1/3 cm, are separated by 10.0 cm. An object of height 6.0 cm is placed 20.0 cm in front of the first lens. What are the size and orientation of the final image?

    A. 6.0 cm, upright
    B. 6.0 cm, inverted
    C. 24.0 cm, upright
    D. 24.0 cm, inverted
    E. 1.5 cm, inverted




    this is my work :
    f1 = 40 cm
    ho = 6 cm
    do = 20 cm

    1/f = 1/di + 1/do
    1/40 = 1/di + 1/20
    di = -40

    f2 = 33.3 cm
    do = 10 cm

    1/f = 1/di + 1/do
    1/33.3 = 1/di + 1/10
    di = -14.29

    m = -di / do
    m = 14.29 / 10
    m = 1.429

    m = hi / ho
    1.429 = hi / 6 cm
    hi = 8.574



    clearly, thats not one of the given options :(

    can someone pleease tell me wat im doing wrong ?
    thnxx
     
  2. jcsd
  3. Jan 16, 2008 #2
    nobody ???
    prettyyy pleaaaseee :(
     
  4. Jan 16, 2008 #3

    hage567

    User Avatar
    Homework Helper

    The image due to the first lens acts as the "object" for the second lens. So once you know where the first image is, you must determine its distance from the second lens. This distance will become your "do" for the second lens and allow you to solve for "di". Once you have these numbers, you can find the size of the final image.
     
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