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Question with Three Thin Lenses Calculating for Final Image Distance

  1. Sep 26, 2014 #1
    1. The problem statement, all variables and given/known data
    Three thin lenses, each with a focal length of 40.0 cm, are aligned on a common axis; adjacent lenses are separated by 52.0 cm.

    Find the position of the image of a small object on the axis, 80.0 cm to the left of the first lens.

    2. Relevant equations

    1/s + 1/s' = 1/f

    3. The attempt at a solution
    So I started this question out with doing:
    1/80 + 1/s' = 1/40
    solving for s' = 80.
    However, my image ends up being between lens 2 and lens 3, 28 cm from lens 2 and 24 cm from lens 3. Now I'm stuck because I don't know what to do next. Any suggestions would be appreciated, thank you in advance.
     
  2. jcsd
  3. Sep 26, 2014 #2

    vela

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    Familiarize yourself with the sign convention used in lens calculations.
     
  4. Sep 26, 2014 #3
    Hmm, but how do I know if it's a converging or diverging lens? I know that for a converging lens, focal length is positive and for a diverging one the focal length is negative. I assumed that the object is real so the distance s would be positive. However, the question doesn't state what type of lens I'm working with.
     
  5. Sep 26, 2014 #4

    vela

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    You're going to need to assume the type of lens you have here, otherwise you're going to have to consider 8 possible cases. Since the problem says the focal length is 40 cm and not -40 cm, I think you're safe going with converging lenses.

    The sign conventions for the object and image only have to do with their positions relative to the lens. Don't worry about whether they're real or virtual.
     
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