We show that the speed of a longitudinal-extended, elastic (variable length), and massive particle, emitted by a source during an emission time T, at speed u (escape speed from all the masses in space), is invariant for every real measurement, (intending a measurement requiring an interaction light-matter), in spite of any reciprocal motion source-Observer. Thus we may argue that the light has to be composed of such particles (photons) moving at speed c = u. Compliance of these photons with Newtonian mechanics is shown for many effects, (like the Doppler effect, redshift, time dilation, etc.), highlighting the differences versus the Relativity. In the 2nd part, on the assumption that the electron charge can be considered as a point-particle fixed to the electron surface, always facing the atom nucleus during the electron revolution, we revised the light-matter interaction, showing that it only depends on the particular impacts between these photons and the circling electrons: for instance, on H atom, we found 137 circular orbits only, the last one being the ionization orbit, where the electron orbital speed becomes vi= c/1372. [Classical mechanics implies that orbiting electrons produce an electro-magnetic radiation causing their fall into the nucleus: on Section 3.5, the reason why the electron circular orbits are stable].