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Convex Lens and Magnification

  1. Apr 7, 2015 #1
    1. The problem statement, all variables and given/known data
    You wish to obtain a magnification of -2 from a convex lens of focal length f. The only possible solution is to:

    A) place a virtual object at a distance 2f/3 from the lens.
    B) place a virtual object at a distance 5f/3 from the lens.
    C) place a real object at a distance 3f/2 from the lens.
    D) place a real object at a distance 2f/3 from the lens.
    E) place a virtual object at a distance 3f/2 from the lens.

    2. Relevant equations
    (1/do) + (1/di) = (1/f)
    m = -di/do
    f is negative because it is a convex lens

    3. The attempt at a solution
    I'm not sure how to factor in focal length, I am unsure how to get the relationship between object distance and focal length
     
  2. jcsd
  3. Apr 8, 2015 #2
    Sylvia, you can attempt this quetion by eliminating approach.
    Here's what you should consider,
    First try by using real objects.
    Note that for magnification of -2 you should have an inverted image.
    We kniw for object between f and o , the image is virtual and erect hence,
    Option D is ruled out.
    Now considering option C
    The objet will be between f and 2f
    Hence image will be real and enlarged.
    And also by putting magnification ( -v/u = -2) you can get your answer.
    Hence option C seems correct to me.

    If you want further solution letme know.
     
  4. Apr 8, 2015 #3

    ehild

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    Homework Helper
    Gold Member

    The focal length of convex lenses is positive.
    You know how magnification defined. So you can express do in terms of di and substitute into the equation (1/do) + (1/di) = (1/f). Solve for do in terms of f.
     
  5. Apr 8, 2015 #4
    Thank you both very much, I understand it now!
     
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