Bohr model (1913) depiction of a photon being emitted as a result of the downward movement of an electron in atomic orbital structure.

In science, a photon is a particle of light. [1] About 1,000 billion photons of sunlight fall on a pinhead each second. To move an electron down in a valence shell orbital of an atom or molecule requires the release of a photon; whereas, to move an electron up in a valence shell orbital requires the absorption of a photon. This is called the Bohr model, a theory developed in 1913 by Danish physicist Niels Bohr.

Evolution
The impact of photons from the sun, thus forcing valence shell electrons upwards into higher energy atomic orbitals, aside from the action of gravity, is the central mediator (force carrier) or driving force of evolution. In the dynamics of the human molecule case, approximately 80% of sensory input (force) is visual and approximately a minimum of 4-5 photons are required to generate a nerve impulse, thus mediating human behavior (induced movement). [5]

History
Building on the 1830s work of who had built on the work English chemist and physicist Michael Faraday and his conception of "lines of force" (or field lines), Scottish physicist James Maxwell, through the publication of his 1873 Treatise on Electricity and Magnetism, had established that light or all forms of electromagnetic radiation consisted of electromagnetic waves. Subsequently, by 1905 the wave nature of light was an established, incontrovertible fact.

In 1901, German physicist Max Planck introduced the notion of the energy element.

In 1905, German-born American physicist Albert Einstein, however, proposed that light was not continuous but consists of localized particles: [2]

“According to the assumption to be contemplated here, when a light ray is spread from a point, the energy is not distributed continuously over ever-increasing spaces, but consists of a finite number of energy quanta that are localized in points in space, move without dividing, and can be absorbed or generated only as a whole.”

Etymology
These “energy quanta” were later terms labeled as “photons” a 1926 coinage of American physical chemist Gilbert Lewis, who, in his “The Conservation of Photons” Nature article, describe a particle of light, as follows: [3]

“I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon.”

Another sources, e.g. Isaac Asimov, claims that soon after Einstein’s papers appeared, that American physicist Arthur Compton coined the term photon in his 1923 work on the investigations showing that electromagnetic quanta behave like particles, exchanging both energy and momentum in collisions with electrons. [4]

References
1. Gribbin, John. (2000). Q is for Quantum – An Encyclopedia of Particle Physics. New York: Touchstone Books.
2. Einstein, Albert. (1905). “On a Heuristic Point of View about the Creation and Conversion of Light”, Annalen der Physik March 18.
3. (a) Lewis, Gilbert. (1926). “The Conservation of Photons” (abs), Nature, 118(2981): 874-75.
(b) Ball, David W. (2001). The Basics of Spectroscopy (pg. 13). SPIE Press.
(c) Gribbin, John. (2000). Q is for Quantum – An Encyclopedia of Particle Physics. New York: Touchstone Books.
4. (a) Asimov, Isaac. (1966). The Neutrino, Ghost Particle of the Atom. Doubleday.
(b) Schumm, Bruce. A. (2004). Things Down Deep – the Breathtaking Beauty of Particle Physics (pg. 33). Baltimore: The Johns Hopkins University Press.
5. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview), (Google books). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (Google books). Morrisville, NC: LuLu.

External links
● Photon – Wikipedia.