A unified description of the physics of the photon based on the transverse vector potential is presented. The description covers subjects from microscopic quantum electrodynamics (QED) to eikonal theory in curved space-time. The link between photon wave mechanics and QED is emphasized. Localized transverse single-photon states in direct space are introduced via a new mean position state in Hilbert space. It is shown that the related correlation matrix is proportional to the transverse subspace part of the Feynman photon propagator. It is argued that a transverse photon acquires an effective rest mass in its interaction with charged massive particles. Starting from the Lorenz gauge four-potential form of the Maxwell-Lorentz equations in General Relativity, the eikonal theory for transverse (unlocalized) light particles is discussed. Contact to the geodesic equation for light rays is made.