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Homework Help: Convex Mirrors and Magnification

  1. May 24, 2009 #1
    1. The problem statement, all variables and given/known data
    What is the image distance for an automobile 4.5 m in front of a convex miror with a 0.15 m focal length? I found this to be 0.155 m

    The question I'm having a problem with is: What is the magnification?

    2. Relevant equations

    magnification = - (image distance/object distance)

    3. The attempt at a solution

    I thought it would simply be,
    -[0.155m/4.5m] = -0.0344. But it's not.

    Some help would be much appreciated
    Last edited: May 24, 2009
  2. jcsd
  3. May 24, 2009 #2

    Doc Al

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    Staff: Mentor

    Careful with signs. For a convex mirror, what sign will the focal length have?
  4. May 24, 2009 #3
    I realized that because it is a convex mirror, the focal length would be negative. However the marking program marked my answer right. I was confused by this :confused: And because of this discrepancy, I am unsure of how to find the magnification.
  5. May 25, 2009 #4


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    Staff Emeritus
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    Is the image real or virtual? (And therefore, is the image distance positive or negative?)
  6. May 25, 2009 #5

    Doc Al

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    Staff: Mentor

    Then how did you get an image distance of +0.155 m ? (See Redbelly98's comment.)
    Now I'm confused. Your first post seemed to indicate that you knew your answer was wrong.
    You find it just as you stated in your first post:
    But you need the correct image distance (correct in both magnitude and sign), of course.
  7. May 26, 2009 #6
    ok.. i got it. Turns out there was an error/glitch with the marking program. I got the answer in the end. Thanks for everyone's help! :biggrin:
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