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Homework Help: Focal Length given M and distance between obj and img

  1. Dec 3, 2017 #1
    1. The problem statement, all variables and given/known data
    The distance between an object and its upright image is 28.0 cm. If the magnification is 0.550, what is the focal length of the lens that is being used to form the image?

    2. Relevant equations
    1/f = 1/so + 1/si
    M= -si/so

    3. The attempt at a solution
    See included image.

    I've been having a hard time with this all day any help would be amazing.

    Can't seem to get the right answer.

    Attached Files:

  2. jcsd
  3. Dec 3, 2017 #2


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    Gold Member

    Ok, there is one problem with your diagram. The image is on the wrong side of the lens. The object is a certain distance, s0, from the lens on the left side. The image is a certain distance, si, from the lens on the opposite side.

    In your diagram, si + 0.28 m = s0. But you must remember that si is negative. Look back at your equation for magnification: m = -si/s0. The s0 is positive, so the distance for the image, si, should be negative in order to produce a positive magnification.

    I hope that helped. Just try and do your same equations, but in the diagram, remember that the distance for the image in the equation is negative. Then, just list down all the equations you have, and you should see a simultaneous equation.
  4. Dec 3, 2017 #3
    thanks! ill give that a try
  5. Dec 3, 2017 #4


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    Gold Member

    You're welcome.
  6. Dec 3, 2017 #5
    Solved it! Thanks again!
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