Quantum electrodynamics describes most of the phenomena of the physical world, except for gravitational and nuclear phenomena. All these phenomena can be essentially deduced from 3 basic actions, each of which occurs with a certain probability that can be calculated using the tools of the theory: (1) a photon goes from point to point, (2) an electron goes from point to point, and (3) an electron emits (or absorbs) a photon. Qualitatively, the theory boils down to photons, which make up the light, going from one electron to another, and electrons, which make up the matter, picking up and giving off photons while moving about.
Refraction and Reflection
Both refraction (transmission) and reflection are the result of the interaction of light and matter, that is, of photons with electrons in the atoms and molecules inside the refractive and/or reflective medium.
We learned in Chapter 1 that light changes speed and direction when moving from one medium to another. However, the individual photons do not go slower inside the material. Slowing of the light is caused by the electrons throughout the material scattering the photons, and the degree to which there is scattering is called the refractive index for that particular medium. Similarly, light is not reflected off a surface. In reality, the photons are scattered by the electrons in the material, the net result of which is the same as if the photon hit and was reflected by only the surface.
Here’s what essentially happens during refraction and reflection: a photon arrives from the outside, hits an electron, and is picked up (absorption) by the electron; the electron continues a bit, and then a new photon is emitted. This dance of photons and electrons is called scattering of light. Thus, in refracted and reflected light, the photon that emerges from the process is not the same photon as the one that went in. Thus, the behavior of light in classical optics reflects the net result rather than the actual path of the photon, providing a convenient approximation to describe the phenomena we are familiar with.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.