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Optics question

  1. Dec 14, 2004 #1
    Hi, could someone tell me what they get for the focal length of this question:

    A diverging lens produces an image 10.0cm from the lens when the object is placed 30cm from the lens. Calculate the focal length and magnification of the lens.

    *I got -7.5cm, but apparently it's supposed to be -15cm. What am I doing wrong? :surprised
  2. jcsd
  3. Dec 14, 2004 #2

    1/so + 1/si = 1/f
    1/30 - 1/10 = 1/f
    (10-30)/(300) = 1/f
    -1/15 = 1/f
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