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Homework Help: Focal Length Problem

  1. Nov 16, 2014 #1
    1. The problem statement, all variables and given/known data
    A lens made of glass has one concave side with a radius of curvature of 5 cm, and one convex side with a raduis of curvature of 7.5 cm. What is the focal length of the lens? Is this a converging or diverging lens?

    2. Relevant equations
    R1=5 cm
    R2=7.5 cm

    3. The attempt at a solution
    1/f=(1.5-1)[(1/5cm)+(1/7.5cm)] = .5(7.5cm+5cm/.375cm) = 16.667
    So f=1/16.667=.06cm

    The answer is supposed to be -30 cm and diverging. I have done a lot of similar problems right, I don't understand what I keep doing wrong on this one.
  2. jcsd
  3. Nov 16, 2014 #2

    Simon Bridge

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    Check the sign convention.
  4. Nov 16, 2014 #3
    I thought it had something to do with that, but I honestly don't know where to change the sign. Is R2 supposed to be -7.5 because it is convex?
  5. Nov 16, 2014 #4
    I'm getting 30 cm now, but this answer is supposed to be negative. Why would my answer come out positive?
  6. Nov 16, 2014 #5


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    Concave means negative radius and convex means positive one.
  7. Nov 17, 2014 #6

    Simon Bridge

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    ... impossible to say without you show your calculation.
    You probably misplaced a minus sign...
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