## 1 Introduction  
One of the most appealing features of geometrical optics, and also a remarkable and convenient advantage, is perhaps the fact that this model relies on the well-defined and very intuitive concept of ray. This concept, which simplifies the description and analysis of optical processes (e.g., imaging) and phenomena (e.g., reflection, refraction, miracles), has a direct physical meaning, as the path along which the electromagnetic energy in the form of light flows. Accordingly, it has helped us to understand how light goes from one place to another in a simple fashion, so it is not strange that descriptions and methodologies based on the notion of ray have become a valuable tool, for instance, to infer properties of the medium traversed by light (e.g., refractive index, curvature, thickness, etc.) or, conversely, based on such properties, to determine how geometrical arrangements of optical devices (e.g., lenses, mirrors, prisms, etc.) can be devised to act on or control light. However, when we move to the realm of electromagnetic or wave optics, this concept blurs up. That is, rays are still considered, but typically in a virtual sense, as an auxiliary tool to evaluate relatively complex integrals, as it is the case, for instance, to explain the emergence of diffraction and interference phenomena. Accordingly, constructive and destructive interference are associated with path differences with a certain value. The same idea, transferred to the domain of quantum mechanics, actually constitutes the formal basis of the well-known [PERSON] path-integral formulation [1]. Such an imbalance between geometrical and wave optics can be overcome, though, by generalizing the idea of physical ray to the nonzero wavelength limit, beyond the domain of the geometrical optics. A convenient starting point to this end can be a formal extension of the eikonal approach to the nonzero wavelength limit [2]. Nonetheless, there is also an alternative route, which consists in making an effective transfer of concepts from the Bohmian formulation of quantum mechanics [3,4] to optics. This quantum formulation allows us to understand the evolution of quantum systems in terms of probability streamlines or trajectories, which denote paths along which probability flows and that, when are statistically considered, reproduce on a bona-fide event-by-event basis the same results rendered by any other quantum formulation [5]. Although the concept of probability is out of the scope of classical electromagnetism (at the level of [PERSON]'s equations), it is still possible to find a beneficial correspondence between elements from this theory and concepts typically involved in the Bohmian formulation. Actually, this can readily be done if the role of the probability density is identified with the electromagnetic energy density, and the quantum density current or quantum flux [6] with the Poynting vector [7, 8, 9]. This prescription, where the corresponding electromagnetic streamlines or rays describe the paths along which (electromagnetic) flows, allows to describe the wave phenomena accounted for by [PERSON]'s equations on an event-by-event basis [8, 10, 11, 12, 13] in compliance with what one experimentally finds in low-intensity experiments [14, 15], also facilitating the understanding of the statistical results typically obtained in quantum optics in the large photon-count limit [16] without the need to involve Fock states in the description. This alternative formulation is actually not that far from the standard formulation of classical electromagnetism, where the corresponding continuity equation favors the definition of a velocity field relating the electromagnetic energy density with its way to spatially distribute, accounted for by the time-averaged Poynting vector [2]. Apart from the intrinsic applied interest that event-by-event formulations have (analogous to ray-tracing in the geometrical optics limit), there are also experimental facts supporting this view. In 2011, [PERSON] et al.