I read above eq. (5.3) in "Optics" by Eugene Hecht
Fermat's Principle maintains that the optical path length OPL will be stationary; that is, its derivative with respect to the position variable will be zero.
I am wondering, how is this derived?
From this post Derivation of Optical Path Length we know that
$$cT=ns$$ where $c$ is the speed of light, $T$ is time light takes to travel through the medium, $n$ is refractive index, and $s$ is the distance Light have traveled through the medium. What is the meaning of position variable in here? Is it just $x$ position? If it is, then does he means that $$\frac{dcT}{dx}=0 ?$$ Then what is the meaning of this equation, I can't see its relation with Fermat's principle.
