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ttzhou
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Homework Statement
In Sears' Optics, chapter 4, Sears claims that the first and second focal lengths (distance from first and second focal points to the first and second principal points, respectively) are equal, and he seems to imply that this is true in general, without proof.
I am a mathematician at heart, and the lack of proof of this bothers me immensely. Would any PF'ers be able to shed some light on this matter? It would be greatly appreciated. I did a forum search and went through about 4-5 pages and found nothing similar.
Homework Equations
Basically, [itex]f = f'[/itex] where [itex]f[/itex] stands for the first focal length and [itex]f'[/itex] stands for the second.
The Attempt at a Solution
I tried imagining this using Fermat's principle of reversibility, but it seems kind of sketchy...