Applied Physics and Mathematics Annotation << Back WHAT EXACTLY IS A PHOTON? THE NATURE OF RADIATION QUANTA AND COSMOLOGICAL REDSHIFT. PART 3 A.V. KUKUSHKIN In the third part of the article the wave function of the photon is reconstructed using the relativistic laws of classical physics. The model assumes that the EM field of the photon is split into two parts of different nature. The region of existence of the non-Maxwellian field that carries the main part of the energy is the space within two parallel fine photon beams. Around them the external transverse Maxwell field is located. The relations of the vectors of both fields at the boundary of the beams are established with the help of Einstein's assumption (1909) that the external Maxwell field to the "special points" on the plane of equal phases in the form of transverse sections of the photon beams are as the field to its source. The constructive potential of this hypothesis is that it helps construct a model in which the main part of the photon energy is concentrated in two beams in the form of a finite train of harmonic waves with the frequency ω of a Maxwell-like field. The requirement of finiteness of the energy flux of the external Maxwell field through an infinite area of the plane of equal phases is satisfied under the condition that the oscillation of said two trains be antiphase. The amplitude of this wave is determined based on the condition that the sum total of energy of the external and internal fields of the photon is the photon’s total energy which is in the left-hand part of the well-known formula . The resulting expression for the amplitude will be in direct proportion to ω. The latter suggests an explication of the nature of the cosmological redshift, since the decrease in the amplitude due to the energy loss of the photon brought about by the interaction of its external Maxwell field with charged particles of the interstellar medium will cause the immediate drop in its frequency. In addition, the wave model of the photon agrees with the quantum optics experiments since in this model a photon can move simultaneously along two different paths after its partial reflection and passage through an inclined semi-transparent mirror. From a theoretical perspective, it is preferable to assume that, on the contrary, the bulk of the energy is transferred by the photon's external Maxwellian field. Arguments in favor of this assumption are presented. Keywords: photon’s internal Maxwell-like field, photon’s external Maxwell field, classical model of the photon wave function, photon rays, free photon’s trajectory, collision-free interaction of a photon with charged particles of the interstellar medium, cosmological redshift. DOI: 10.25791/pfim.06.2025.1345 Pp. 18-54.