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Radius of curvature ?

  1. Mar 20, 2008 #1
    What is radius of curvature and why is it equal to 2f where f is the focal length of a lens or a mirror ?
  2. jcsd
  3. Mar 20, 2008 #2
    You can check the definition in this : ttp://mathworld.wolfram.com/RadiusofCurvature.html

    The radius of a spherical mirror is equal to 2f : that's right. But the lense : I am not sure.
    You can see in the attached picture: AOF is an isosceles triangle.

    Attached Files:

  4. Mar 20, 2008 #3
    but looking at th diagram I don't see where the centre of that arc is.
  5. Mar 20, 2008 #4


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    Staff: Mentor

    For a concave mirror see


    For a lens, the radius of curvature (it can be different for the front and back surfaces of a lens) is generally not equal to 2f. For the focal length of a lens see



  6. Mar 20, 2008 #5
    What is the significance of radius of curvature ?
  7. Mar 20, 2008 #6


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    Staff: Mentor

    Imagine that the surface is part of a circle or sphere. The radius of curvature is the radius of that circle/sphere.
    Last edited: Mar 20, 2008
  8. Mar 20, 2008 #7
    In my picture, the arc center is O because it is a part of a cicle. Remember that diagram and thurs formula only fits for a spherical concave mirror.

    PS. Sorry I can not login my nick pixel01 because I forgot the pw and unfortunately the email address has been expired.
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