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Homework Help: Mirror reflection question

  1. Nov 1, 2006 #1
    I'm studying for a test that is going to cover mirrors and lenses. I was under the impression that the focal length for any concave or convex mirror was half the radius of curvature:

    but one of the questions in my book involving a convex mirror uses a different value for f, according to the solutions manuel. It says that:

    of all the questions i've encountered while studying for this test, f has always been r/2. can someone help explain why f can be 2/r?
  2. jcsd
  3. Nov 1, 2006 #2


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    Staff: Mentor

    That does seem like a typo. As r gets bigger, f will grow. Can you state the exact problem from the text?
  4. Nov 1, 2006 #3
    So was that a typo? Test tomorrow...
  5. Nov 1, 2006 #4


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    Science Advisor
    Homework Helper

    Yes it was a typo. They meant to say 1/f = 2/r and that is the way they used it in the problem. The right hand side of the next to last line replaced the 1/f with -2/20yd
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