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Find the image distance - optical instruments

  1. Dec 15, 2008 #1
    1. The problem statement, all variables and given/known data
    An object is placed at 30 cm in front of a diverging lens with a focal length of 10 cm. What is the image distance?


    2. Relevant equations
    1/f = 1/do + 1/di



    3. The attempt at a solution
    I thought this was a straight-forward question:
    1/di = 1/-f - 1/do
    1/di = (1/-10 cm) - (1/30 cm)
    1/di = -0.066 cm
    di = 1/-0.066 cm = -15 cm

    BUT- the answer provided to me states that the answer is -7.5cm. So what is it that I'm missing here?
     
  2. jcsd
  3. Dec 15, 2008 #2

    mgb_phys

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    Slight calculator typo
    (1/-10) - (1/30) = - (1/10 + 1/30) = -0.133
     
  4. Dec 15, 2008 #3
    Well now... don't I feel silly?

    Thank you.
     
  5. Dec 15, 2008 #4

    mgb_phys

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    It's very easy to get those sort of sums wrong,
    It's worth rearranging them so that you know if the answer you expect is +ve or -ve
    And in a form where you can estimate the magnitude, eg (1/10+1/30) is obviously going to be a bit bigger than 1/10
     
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