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Optics did my professor screw up?

  1. May 9, 2007 #1
    1. The problem statement, all variables and given/known data
    A cat is standing 1m in front of a mirror. a virtual image is formed 10.0m behind the mirror. what is the radius of curvature for the mirror?

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

    3. The attempt at a solution
    (1/1m + 1/10m)^-1 = .909
    .909*2 = 1.82

    According to my answer key, R should be equal to 2.2m. the only way i get that answer is if i take (1/do)+(1/di) and not take its reciprocal...i would just plug it straight into the focal length equation to solve for R. Did he make a mistake??
  2. jcsd
  3. May 9, 2007 #2


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    The image is virtual so di should be taken to be negative.
  4. May 9, 2007 #3
    you miss substitution
    di = -10.0 m :biggrin:
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