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Determining the object distance from a magnifying glass

  1. Apr 15, 2014 #1
    1. The problem statement, all variables and given/known data
    A magnifying glass has a focal length of 8.0 cm. How close to an object do you need to place the magnifying glass so that the distance between the image and the magnifying glass is the same as the focal length? What type of image is this and what is its magnification?

    2. Relevant equations

    I am thinking that you use the Lens/Mirror equation

    1/focal length = 1/object distance + 1/image distance

    3. The attempt at a solution
    I am unsure of how to do this because I am after object distance. So what I did was 1/object distance = 1/focal length - 1/image distance. But when I do that they just cancel because focal length and image distance are supposed to be the same thing. Once I figure that out I can handle the image type and magnification.

    Thanks, ChuggZ
  2. jcsd
  3. Apr 15, 2014 #2


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    Gold Member

    What you have done is right. Don't stop just because they cancel out, continue and solve for what you want.
  4. Apr 16, 2014 #3


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    Homework Helper

    The image can appear at both sides of the lens. You know only the distance of the image from the lens, but the sign of the image distance can be both positive and negative.

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