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The question can be found here:

The refractive index it references can be found here:

I'm not really sure where to get started on the "showing" part of the question. I anticipate that Snell's Law will be used somehow. I'm not looking for a full answer, just a push in the right direction.

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closed as too localized by Qmechanic Feb 15 '13 at 15:30

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Hi John - as I mentioned on your previous question, please don't use an external link just to show the question. Could you type the contents of the question into your post here? – David Z Sep 8 '12 at 3:48
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You should use the fact that if it is a lens, all the paths to the focal point from a plane perpendicular to the x-axis at some distance behind the lens will take the same time to cross. You can approximate the paths as straight for small deflections, and the extra time from the varying index has to compensate for the extra length from the pythagorean theorem to the focal point.

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