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Homework Help: Focal Distance

  1. Aug 10, 2008 #1
    1. The problem statement, all variables and given/known data
    A converging lens (convex lens) creates an image that is 2 times the size of the object. The object is placed:

    2. Relevant equations

    -di/do = hi/ho
    1/f = 1/di + 1/do

    3. The attempt at a solution

    I thought the answer was that the object is place two focal lengths in front of the lens. But the answer says its half a focal length in front of the lens.

    I mean: -di/do = hi/ho = 2
    -di = 2do
    1/f = 1/di + 1/do
    = -1/2do + 1/do = 1/2do
    f = 2do

    Isn't f=2do saying two focal lengths?
    Last edited: Aug 10, 2008
  2. jcsd
  3. Aug 10, 2008 #2


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    Homework Helper

    I'm not sure what you mean. If [itex]f=10[/itex] for example, then [itex]d_o=5[/itex], so [itex]d_o[/itex] is half the focal length.
  4. Aug 10, 2008 #3
    So simple, thanks. I guess I just missed an easy answer by thinking too much (or thinking to less).

    Thanks again
  5. Aug 10, 2008 #4


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    Staff Emeritus
    Science Advisor
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    f=2do is saying do=f/2, or half a focal length.

    (do=2f would be saying two focal lengths.)
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