# Doubt regarding Fermat's principle [duplicate]

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Which two points are we talking about in Fermat's principle? Are those points decided by light or decided by us? Can we take any two points?

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• Fermat's principle states that given any two points $P$ and $Q$ between which there is a ray then the optical path length defined by $[OPL]= \int_{P}^{Q}nds$ is extremum among all neighboring paths connecting the fixed points $P$ and $Q$. – hyportnex May 2 '18 at 14:45