Circular light ray in a linear speed profile
= Circular light ray in a linear speed profile
{title2=$(x-x_0)^2+(y+c_0)^2=R^2$}
For speed $c(y)=y+c_0>0$, <Fermat principle> and the <Beltrami identity> yield circular <light rays> whose centres lie on $y=-c_0$. The ray between $(\pm a,0)$ in the upper half-plane is $y=\sqrt{a^2+c_0^2-x^2}-c_0$.