# Optics -- caustic curve equation

http://users.df.uba.ar/sgil/physics_paper_doc/papers_phys/ondas_optics/caustica1.pdf [Broken]
But I think that when we do F=0, we are assuming the centre of the curve to be the origin and assuming the curve to be a circle, because only then will we be able to say that OP=ON (i.e. F=0)..either the article has some issues or I am interpreting it wrongly..please help..

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Andy Resnick