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Homework Help: Find the change in focal length

  1. Mar 26, 2009 #1
    1. The problem statement, all variables and given/known data
    A nearsighted person has eyeglasses with a -1D prescription. If the near point is at 23 cm without eyeglasses, where is it when they are put on?

    2. Relevant equations

    3. The attempt at a solution
    I got the answer which is 0.298701298 meters by -1= (1/do)-(1/0.23)...but the problem is that i don't know why it's like that...Any ideas would be extremely helpful!! Thanks!!
  2. jcsd
  3. Mar 26, 2009 #2


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    Science Advisor
    Homework Helper

    Re: lenses

    You have that equation wrong,
    f = the focal length of the lens ( -0.23m)
    di is the image distance (which you don't know - but can assume is the same )
    do is the object distance

    Then you need the formula for a combination of two lenses (ie the eye + glasses) to find the change in focal length.
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