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Homework Help: Lenses and Mirrors

  1. Apr 20, 2004 #1

    A concave mirror has a focal length of 62.4 cm.
    Determine the object position for which the resulting image is upright and four times the size of the object.

    What formula do I use for this question? :confused:
  2. jcsd
  3. Apr 20, 2004 #2


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    Homework Helper

    h'/h = -q/p
    1/p + 1/q = 1/f
  4. Apr 20, 2004 #3
    haha...i already knew those formulas.
    but what's stumping me is...i only know f and the magnitude -.-
    is there something that i'm not seeing? i can be totally blind sometimes!
  5. Apr 20, 2004 #4


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    Homework Helper

    I don't understand. What is "-.-"? Try drawing a ray diagram to help you visualize. Beware of the sign conventions for the concave mirror (which I don't recall off the top of my head).
  6. Apr 20, 2004 #5
    What is the answer meant to be?
  7. Apr 21, 2004 #6

    Doc Al

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    Staff: Mentor

    You have all the info needed to solve this problem, you're just not putting things together. The fact that the image is upright means that the linear magnification is positive: in this case m = +4. Now use the definition of m to write the object distance (p) in terms of the image distance (q). Then when you plug into the mirror equation, you can solve for p.
  8. Apr 27, 2004 #7
    yup because
    m = distance of image/distance of object so we can say that m = 4 hence
    di = 4 x do (where i and o represent image and object)

    Then we can plug that into the equation for focal length which is
    1/di + 1/do = 1/f
    and we know the f, so we can solve for 'do' to get the answer :)

    thanks very much.
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