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Location of object on axis of concave mirror?

  1. Aug 9, 2017 #1
    1. The problem statement, all variables and given/known data
    Where on the axis of a concave mirror would you place an object to get a half-size image? Express the object distance in terms of the focal length f. Follow the sign convention.

    2. Relevant equations
    (1/q)+(1/p)=(1/f)
    f=(2/R)

    3. The attempt at a solution
    I attempted to use the object distance plus image distance to solve for f (I just got f=q+p), but the online hw system says that is wrong. Are these the correct formulas and what should I be doing?
     
  2. jcsd
  3. Aug 9, 2017 #2

    Merlin3189

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    Those equations don'tmention image size. So you need an idea about that, or maybe an equation.
    Have you drawn a diagram?
     
  4. Aug 9, 2017 #3
    But they do? I wrote them differently, but it's (1/Do)+(1/Di)=(1/f)

    EDIT: wait I misread that. You are talking about (H'/H)=(f/k)
     
  5. Aug 10, 2017 #4

    Merlin3189

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    Yes. I don't recognise the formula you give (H'/H)=(f/k) ?
    But it's the ratio of heights (H'/H) that matters. You know what number this is, from the question.
    How do the heights relate to the distances ? (That's where I thought a diagram might help.)
     
  6. Aug 10, 2017 #5

    Merlin3189

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    concave_mirror.png
     
    Last edited: Aug 10, 2017
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