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Homework Help: Thin lens equation for diverging lenses.

  1. Oct 17, 2007 #1
    Hi, I was reading a section which explains about the
    "thin lens equation for diverging lenses" and it included an example;

    1/f = 1/di + 1/do
    where f is the focal distance,
    di is the distance of the image from the center and,
    do is the distance of the object from the center.

    f = -4cm
    do = 3cm
    di = ???

    1/di = 1/f - 1/do
    1/di = -1/4cm - 1/3cm
    1/di = -3/12cm - 4/12cm
    1/di = -7/12cm
    1/di = -12/7cm = -1.7cm

    The part where I have a confusion is colored in red.
    I've been questioning, why -7/12cm was flipped in the last step.

    If that example was to be solved as decimal numbers it would be;
    1/di = -0.250cm^-1 - 0.333cm^-1
    1/di = -1/0.583cm
    then in this case, I would understand why the number has been reversed because it was ^-1
    but when solving in fraction, I get confused..

    Could anybody please explain for me?
    Thank you in advance
    Last edited: Oct 17, 2007
  2. jcsd
  3. Oct 18, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I assume it's a typo. That last line should have read:
    di = -12/7cm = -1.7cm

    I assume that you understand that: 1/x = a/b can be inverted (as long as you're not dividing by zero) to become x = b/a.
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