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Find the focal length of a concave lens using a convex lens?

  1. Jun 11, 2008 #1
    Can anyone explain to me the procedure that one would follow to you find the focal length of a concave lens using a convex lens? Thanks.
  2. jcsd
  3. Jun 12, 2008 #2
    Yeah there is a way

    Assume the convex lens creates an image of S at the point S'. Now if you put a concave lens in front of the convex one then the image of S will move further and will be at point S''. Now consider a reverse path of light: S'' is now the object and you'll see it's image at S' which is created by the concave lens. Let the distance from the "center" of the concave lens to S'' be a and the distance from S' to the "center" of the concave lens be b. Then using the lens formula:

    [tex] -\frac{1}{f}=\frac{1}{a}-\frac{1}{b} [/tex] notice there is a minus near b and f. It's because the image is imaginary


    [tex] f=\frac{ab}{b-a} [/tex]

    I hope I didn't make any mistakes
  4. Jun 12, 2008 #3


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    erm, armis … you're doing it again!

    If you'd changed "Then using the lens formula:" to "Then use the lens formula! :smile:", and stopped there, then that would have been a good hint. :wink:

    (The OP can always come back and ask for more help, if necessary, of course :smile:)
  5. Jun 12, 2008 #4
    Oh... I thought I was supposed to do that only in the homework section, sorry. But it does make a lot more sense your way. I think I finally got the idea
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