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Homework Help: Finding object distance with converging lenses.

  1. May 3, 2009 #1
    A converging lens with f=33.7 cm yields an image with magnification M=−2. Find the object distance. Give your answer ...

    * in terms of centimeters. Do not include units in your answer. They will be assumed to be cm.
    I'm not really sure how to approach this problem. I keep calculating it and getting the wrong answer. I started with the 1/f =1/do + 1/di equation, and know that M = -di/do, but from there my work is probably wrong

    -2=-di/do : -2do = -di ----> 2do=di

    1/f=1/do + 1/2do

    1/f =1/3do

    f= 3do
    1/3 * 33.7cm =do
    do= 11.23cm =WRONG.
  2. jcsd
  3. May 3, 2009 #2
    Ok, your mistake is going from the 2nd line to the 3rd line.

    You didn't add the fractions properly. What is 1+1/2?
  4. May 4, 2009 #3
    I'm still confused... 1/f= 3/2do? i don't know where to go from there
  5. May 4, 2009 #4
    Solve for 'do' since you know what 'f' is.
  6. May 4, 2009 #5
    thanks! i got the right answer this time....
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