If the direction of propagation is always such that we can use instead of as the parameter of the path (and the overdot to denote differentiation w.r.t. instead of ), the optical Lagrangian can instead be writtenCf. Chaves, 2016, p.577.
If the direction of propagation is always such that we can use instead of as the parameter of the path (and the overdot to denote differ.entiation w.r.t instead of ) the optical Lagrangian can instead be writtenCf. Chaves, 201,6, p.577.