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Concave Mirrors

  1. Mar 15, 2010 #1
    1. The problem statement, all variables and given/known data
    1)A concave mirror produces a virtual image that is three times as tall as the object.
    a) If the object is 21 {\rm cm} in front of the mirror, what is the image distance?
    b) What is the focal length of this mirror?

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

    3. The attempt at a solution
    hi/ho = 3 thus do/di = 3 a) di = -do /3 = -7.0cm (was wrong)
    thus i could not do the second part. Where did I go wrong? And if it was a real image instead of a virtual image, what what you do different? Thanks
  2. jcsd
  3. Mar 15, 2010 #2


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    Hello matt72lsu,

    hi/ho and do/do are not really equations. If you put these in full equation form, ensuring that they apply to virtual images in concave mirrors, it might help.

    I believe you've got your do and di switched around.

    For one thing, knowing whether the image is real or virtual gives you information about where the object is relative to the mirror's focal length (is the object between the focal length and the mirror or is it on the far side of the focal length?).
  4. Mar 17, 2010 #3
    i'm sorry, i just have no clue what to do. the whole 3 times larger part is throwing me off
  5. Mar 18, 2010 #4


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    If the image is 3 times as large, than it's 3 times as far away. :wink:

    Start by going back to your original equations.

    Earlier you calculated
    But that should be di/do = 3.
  6. Mar 18, 2010 #5
    i got this one already
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