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Homework Help: Converging lens magnification

  1. Sep 29, 2007 #1
    If I have a converging lens, of focal length f, and magnification of the image m, where q is the distance of the image from the lens show that q = (1+m) f

    m = - q/p
    1/f = 1/p + 1/q

    I just need to show that they are equal, but im getting confused with the maths..

    q= (1 - (q/p)) (1/(1/p + 1/q))
    q= (1 - (q/p))(qp/(p+q)
    q= (qp/p + q) - (q/p)(qp/(p+ q)
  2. jcsd
  3. Sep 29, 2007 #2

    Chi Meson

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    Science Advisor
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    If by "they" you mean p=q, you could look at the scene from the ray diagram point of view. The only situation where p=q for a converging lens, is when both are at a point at 2x the focal length of the lens. At this point, the gemoetry is perfectly symmetrical, and image is the same size as the object, therefore, magnification is 1 (w
  4. Sep 29, 2007 #3

    Doc Al

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    Staff: Mentor

    Good. Rearranging, this becomes:
    q= [(p-q)/p][qp/(p+q)] = [(p-q)/(p+q)]q

    Well, looks like it's not true! :eek:

    Perhaps they meant to write: q = (1-m)f
  5. Sep 29, 2007 #4

    Chi Meson

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    Actually I get unity each time I do it (actually 1= -1). So then I noticed that the first equation:
    q= (1+m)f
    is derived directly from the thin-lens and magnification equations, if you use m=q/p (no negative).

    So the equation q=(1+m)f is a general equation (true for all real situations) not a specific equation, as long as you are using the non-negative convention for the magnification equation.

    There is a problem with the question, as stated.
  6. Sep 29, 2007 #5
    thanks so much for all your help, its great.

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