  Optical lossless beam splitters are frequently encountered in fundamental
physics experiments regarding the nature of light, including "which-way"
determination of light particles, N. Bohr's complementarity principle, or the
EPR paradox and all their measurement apparatus. Although they look as common
optical components at first glance, their behaviour remains somewhat mysterious
since they apparently exhibit stand-alone particle-like features, and then
wave-like characteristics when inserted into a Mach-Zehnder interferometer. In
this communication are examined and discussed some basic properties of these
beamssplitters, both from a classical optics and quantum physics point of view.
Herein some convergences and contradictions are highlighted, and the results of
a few emblematic experiments demonstrating photon existence are discussed. An
alternative empirical model in wave optics is also proposed in order to shed
light on some remaining questions
