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Homework Help: (easy) Finding variables with mirror equation

  1. Dec 19, 2012 #1
    This isn't exactly a homework problem, per se, but this was a district assessment I already took today. I was confident that I would get a good grade, but it turns out I got a measly 65%. I don't remember the exact wording, but I do remember the general idea of it.

    1. The problem statement, all variables and given/known data
    There are two lenses, both of which are 16cm apart. An object is 1.3cm away from the first lens.

    First lens
    f = 1.2cm
    do = ?
    di = ?
    M = ?

    Second lens
    f = .42cm
    do = ?
    di = ?
    M = ?

    Total Magnification
    Mtot = ?

    2. Relevant equations
    di = dof/do-f
    M = di/do
    Mtot = M1*M2

    3. The attempt at a solution
    First lens
    f = 1.2cm
    do = 1.3cm (given)
    di = 15.6 cm
    M = 12x

    Second lens
    f = .42
    do = 16cm+1.3cm = 17.3cm
    di = .43 cm
    M = .025x

    Total Magnification
    Mtot = .3x

    Looking back at it now, I think I messed up somewhere in the second lens.
    I've asked friends what they got for Mtot, and the majority of them said a number around the 200s.

    What am I doing wrong?
  2. jcsd
  3. Dec 19, 2012 #2


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    Staff Emeritus
    Science Advisor
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    Gold Member

    Hello classi. Welcome to PF !

    The image formed by the first lens becomes the object for the second lens.
  4. Dec 19, 2012 #3
    Oh my gosh, thanks! :)
    That makes perfect sense; I don't know why I didn't think of that before.
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