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Homework Help: Concave mirrors - where is the object

  1. Nov 10, 2006 #1

    danago

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    Gold Member

    "A concave mirror of focal length 1m is used to magnify an object by a factor of 4. Where is the object in relation to the pole of the mirror?"

    Heres what i did:
    [tex]
    \begin{array}{c}
    M = \frac{v}{u} \\
    \therefore 4 = \frac{v}{u} \\
    \therefore v = 4u \\
    \end{array}
    [/tex]

    [tex]
    \begin{array}{c}
    \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \\
    \therefore 1 = \frac{1}{u} + \frac{1}{{4u}} = \frac{5}{{4u}} \\
    \therefore u = 1.25m \\
    \end{array}
    [/tex]

    Where am i going wrong? According to my textbook, the answer is 0.75m.

    Thanks in advance,
    Dan.
     
  2. jcsd
  3. Nov 10, 2006 #2
    You are forgeting that M = -di/do

    Formula = (1/do + 1/di) = (1/f)

    So 4do = - di

    1/do - 1/4do = 1

    1/4do - 4/4do = 1

    3/4do = 1

    do = .75
     
    Last edited: Nov 10, 2006
  4. Nov 11, 2006 #3
    Right.

    3/(4do) = 1 so do = .75
     
    Last edited: Nov 11, 2006
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