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Homework Help: Index of refraction for converging lens

  1. Mar 13, 2016 #1
    1. The problem statement, all variables and given/known data
    A thin converging lens has radii of curvature of R1=9cm and R2=-11cm. Assume its focal points F1 and F2 are 5cm from the center of the lens. Determine its index of refraction.

    2. Relevant equations
    Len's Makers equation: 1/f = (n2 - n1 / n1) (1/R1 - 1/R2)

    3. The attempt at a solution
    focal length f = 5cm
    n1 = air = 1

    Plugging in all my known values:
    1/5 = (n2 - 1) (1/9 - 1/11)
    n2 = 10.9

    This value for refractive index doesn't make sense at all... so I'm not sure what I'm doing wrong!
  2. jcsd
  3. Mar 13, 2016 #2


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    Did you take into account that R2 is negative?
  4. Mar 13, 2016 #3
    D'oh! What a ridiculous mistake. Yes, with that taken into account the refractive index is 1.99.
  5. Mar 13, 2016 #4


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    OK, good.
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