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Convex Lens

  1. Jul 31, 2011 #1
    1. The problem statement, all variables and given/known data

    I did an experiment with a convex lens, object, and image. As the lens moved farther and farther away from the object, the image decreased its distance (to remain in focus) from the object as well up to a certain point. After that point, as the lens increased its distance from the object, the image had to increase its distance from the object as well to keep in focus.

    Why is this?

    2. Relevant equations


    3. The attempt at a solution

    It's a theoretical question with no exact solution.
  2. jcsd
  3. Jul 31, 2011 #2


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    One could say that it happens that way because that is the way a lens behaves.

    The mathematical summaries of the situations with a lens [the lens formulae] indicate this behaviour - especially if you look at the right formula.

    What lens formulae do you know?
  4. Jul 31, 2011 #3
    1/f = 1/s + 1/s'

    m = h'/h = -s'/s

    Does it have something to do with the focal point?
  5. Jul 31, 2011 #4


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    I would like to say yes to that.
    Do you know the formula using the distances from the focal points rather than from the lens?
  6. Jul 31, 2011 #5


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    Both the object distance and the image distance are measured from the lens. The object and the real image are at opposite sides of the lens, so the object and the image are d=s+s' distance from each other. The image distance is s'=d-s. Plug in for s' into the lens equation, and find d in terms of the object distance s. Investigate the function, plot it. You will see, it has a minimum.

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