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Same focus for all circular lenses?

  1. Oct 28, 2005 #1
    In school physics i have learnt that the focus for a circular convex(converging) lens is f=r/2 where r is the radius and f is focus. But wouldn't a pair of identical lens made of a different material eg glass and perspecs which have different refractive indexes have different point of focus?
     
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  3. Oct 28, 2005 #2

    James R

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    I think the formula f=r/2 only applies for idealised thin lenses. Hopefully, somebody else can provide more detail.
     
  4. Oct 28, 2005 #3

    Doc Al

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    lens maker's formula

    You are thinking of a concave (converging) mirror, not a lens.

    Of course. The focal length of a lens depends on the refractive index as well as the curvature of the surfaces:
    [tex]f = n (\frac{1}{R_1} - \frac{1}{R_2})[/tex]

    [itex]R_1 , R_2[/itex] are the radii of the surfaces. Look up the "lens maker's formula" to understand how this is used.
     
  5. Oct 30, 2005 #4

    James R

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    Thanks, Doc Al. Stupid of me not to realise that.
     
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