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Homework Help: Spherical mirror radius of curvature

  1. Aug 11, 2011 #1
    1. The problem statement, all variables and given/known data

    A dentist uses a spherical mirror to examine a tooth. The tooth is 1.13 cm in front of the mirror, and the image is formed 10.8 cm behind the mirror. Determine the mirror's radius of curvature.

    2. Relevant equations



    3. The attempt at a solution

    Since the object is in front of the mirror, p > 0. With the image behind the mirror q < 0. So the radius of curvature is

    [tex]\frac{2}{R}=\frac{1}{p} + \frac{1}{q} = \frac{1}{1.13} - \frac{1}{10.8} = \frac{10.8-1.13}{10.8}[/tex]

    So [tex]R = 2 \frac{10.8}{9.67} = 2.233[/tex]

    Why is my answer is wrong? I think I used the correct equations. I tried the same question with different numbers and the computer still marks my answer wrong. Any explanation would be appreciated.
  2. jcsd
  3. Aug 11, 2011 #2


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  4. Aug 11, 2011 #3


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    You mucked up the subtraction of two fractions - in particular, the denominator.
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