| at each point, the path tangential to the direction of propagation of energy at this point
NOTE 1 – The concept of ray is the basis of the geometrical optics which, when applicable, permits the substitution of simple relationships for Maxwell's equations.
NOTE 2 – In some cases, several paths may exist between two points.
NOTE 3 – In an isotropic medium, the propagation path is a trajectory orthogonal to the wavefronts and the term "ray" is often defined as this trajectory. In an anisotropic medium, the trajectories orthogonal to the wave fronts do not always coincide with physical paths between a source and a receiving point and should not be called rays.