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Homework Help: Focal length of plano convex lens

  1. Jun 23, 2006 #1
    # Diameter of a Plano-convex lens is 6 cm and the thickness at the center is 3 mm. If the speed of light in the material of the lens is 2 x 10^8 m/sec, what is the focal length of the lens?
    I solved it in the following way:
    Let R be the radius of curvature of the convex surface.
    Given that AB = 3 mm = 0.3 cm, AD = CD/2 = 3 cm, refractive index(n) = 1.5
    By the property of circles,
    AB = (AD^2)/(2R)
    Therefore, R = (AD^2)/(2AB)
    = 9/(2 x 0.3)
    = 15 cm
    According to thin lens formula,
    1/f = (n – 1)/R
    Solving I get,
    f = 30 cm
    Is it right?

    Attached Files:

  2. jcsd
  3. Jun 25, 2006 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    Yes. For the benefit of others, you are using the lensmaker's forumula for thin lenses, not to be confused with the thin lens equation.

  4. Jun 26, 2006 #3
    Thanks for your help.
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