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- Thread starter jias
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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

Thus

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

I hope I didn't make any mistakes

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tiny-tim

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If you'd changed "Then using the lens formula:" to "Then use the lens formula! ",

(The OP can always come back and ask for more help, if necessary, of course )

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