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Focal Length - please check my work and tell me what's wrong.

  1. Apr 8, 2010 #1
    1. The problem statement, all variables and given/known data
    A small object is 25.0 cm from a diverging lens as shown in the figure . A converging lens with a focal length of 11.4 cm is 30.0cm to the right of the diverging lens. The two-lens system forms a real inverted image 17.0 cm to the right of the converging lens.

    3. The attempt at a solution
    1/f = 1/o + 1/i at every step

    conv lens

    1/12 = 1/o + 1/17.0

    o = 40.8

    This means the second object, which is the first image, was 40.8 cm to the left of the conv lens.

    This means the second object was 40.8 - 30 = 10.8 cm to the left of the div lens

    So the image distance for the div lens is -10.8 and

    1/f = 1/25.0 + 1/(-10.8)

    1/f = 0.04 - 0.0926 = -0.05259

    f = -19.0 cm is the focal length of the div lens
     
  2. jcsd
  3. Apr 8, 2010 #2

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    Hello vucollegeguy,

    Where do you get a length of 12 cm? :uhh: The problem statement says that the converging lens has a focal length of 11.4 cm. You might want to correct this before you move on. 'Turns out it makes a significant difference in the end.
     
  4. Apr 9, 2010 #3
    I changed 12 to 11.4. My final answer came out to be -5.6 but distance can't be negative. Is my answer correct?
     
  5. Apr 9, 2010 #4

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    Correct me if I'm wrong, but I thought you were solving for the focal length of the diverging lens. Focal lengths of diverging lenses are negative by definition.
     
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