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Homework Help: Optics: Thin lenses

  1. Dec 2, 2013 #1
    1. The problem statement, all variables and given/known data

    In the photo (which I cannot post now, but it is a magnifying glass, and a dollar bill) the images through the lens appears twice the size and it is at a distance of 3.0cm from the lens of the magnifying glass. Determine focal length.

    2. Relevant equations

    1. -di/do=m
    2. 1/f=1/di+1/do

    3. The attempt at a solution

    Since twice the size m will be 2. Using equation 1:

    = -2*3.00cm
    = -6.00cm

    This makes sense since image is upright and virtual, and in lenses virtual images are negative.

    When I calculate focal length,


    But this answer is wrong the answer is 3. What am I doing wrong or what do I not understand?

  2. jcsd
  3. Dec 2, 2013 #2


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    Perhaps the word "it" here refers to the image rather than the object.
  4. Dec 3, 2013 #3
    The answer is clearly wrong. If the object was placed on the suggested focal point, 3 cm, the image would be real and located at infinity.
  5. Dec 3, 2013 #4


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    You will get the right answer if you interpret the distance of 3.0 cm as the magnitude of the distance from the lens to the image.
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