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This question already has an answer here:

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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marked as duplicate by John Rennie visible-light May 2 '18 at 5:05

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Hi Kiran. While the question I have linked is a duplicate it's quite technical and you my find it impenetrable. If so ping me here and I'll reopen this question so someone can add a simpler answer. $\endgroup$ – John Rennie May 2 '18 at 5:06
  • $\begingroup$ 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 $Q$. $\endgroup$ – hyportnex May 2 '18 at 14:45