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## Homework Statement

In Bessel's method for finding the focal length f of a lens, an object and a screen are separated by distance L, where L is greater than 4f. It is then possible to place the lens at either of two locations, both between the object and the screen, so that there is an image of the object on the screen, in one case magnified and in the other case reduced. Show that if the distance between those two lens locations is D, then the focal length is given by f=(1/4)(L

^{2}-D

^{2})/L.

## Homework Equations

Thin lens equation: [tex]\frac{1}{s}[/tex]+[tex]\frac{1}{s'}[/tex]=[tex]\frac{1}{f}[/tex]

Focal length: [tex]\frac{1}{f}[/tex]=(([tex]\frac{n}{n

_{air}}[/tex])-1)([tex]\frac{1}{r

_{1}}[/tex]-[tex]\frac{1}{r

_{2}}[/tex])

## The Attempt at a Solution

I know that you guys are not able to help me out until I show you my attempt, but I do not even know where to start. I guess if you guys could tell me whether or not these equations are relevant, then I could work from there and get back to you with an actual attempt.

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