ABSTRACT
Photons occupied a unique status in particle physics. A “free” photon has zero mass, which makes the energy and momentum numerically the same value. Early developments in quantum theory incorporated photons as waves, with quantum mechanical representations for the material bodies with which photons might interact. Photon-matter scattering was described very well by the Klein–Nishina formula, which treats photons as monochromatic waves of well-defined energy. One aspect of the mathematics represents the loss of the incoming photon when it encounters the electron. Electromagnetic interactions, whether between two electric charges or involving magnetic properties, are prescribed by the exchange of a virtual photon transported between the two interacting partners. A photon is an exchange particle to the extent that it inherits its properties from its source conditions and then imparts these properties to the object with which it subsequently interacts. The loss of intensity relates to the ongoing interaction of a photon with the constituents of the medium, through absorption.
