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Homework Help: Real or virtual, focal length

  1. Jul 29, 2007 #1
    A positive lens has a focal length of 12cm. An object is located a distance of 3 cm from the lens.
    How far is the lens from the image?
    Is the image real or virtual, erect or inverted?

    My attempt
    f = +12cm
    o = +3cm
    i =?
    1/o + 1/I = 1/f
    1/i = 1/f – 1/o
    = 1/12cm – 1/3cm
    = 1/12 – 4/12
    = - (3/12)
    i = -4cm on the same side of the object.

    Since the object lies in within the focal point, the image is virtual and erect.
  2. jcsd
  3. Jul 29, 2007 #2


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    Science Advisor
    Homework Helper

    Always draw the ray diagram before trying to do the maths.
  4. Jul 29, 2007 #3


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    Staff Emeritus
    Science Advisor
    Gold Member

    Looks good to me.
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