text: The Gladstone–Dale relation requires a particle model of light because the continuous wave-front required by wave theory cannot be maintained if light encounters atoms or molecules that maintain a local electric structure with a characteristic refractivity. Similarly, the wave theory cannot explain the photoelectric effect or absorption by individual atoms and one requires a local particle of light (see Wave–particle duality).
  A local model of light consistent with these electrostatic refraction calculations occurs if the electromagnetic energy is restricted to a finite region of space. An electric-charge monopole must occur perpendicular to dipole loops of magnetic flux, but if local mechanisms for propagation are required, a periodic oscillatory exchange of electromagnetic energy occurs with transient mass. In the same manner, a change of mass occurs as an electron binds to a proton. This local photon has zero rest mass and no net charge, but has wave properties with spin-1 symmetry on trace over time. In this modern version of Newton's corpuscular theory of light, the local photon acts as a probe of the molecular or crystal structure.