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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


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    Homework Helper

    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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