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

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, technically defined as a boson with zero mass, and spin 1. [1] On the surface of the earth, about 1,000 billion photons of sunlight fall on a pinhead each second.

Bohr model
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]

Electromagnetic field
The following is the modern particle physics interpretation of the relation between photons, light, electromagnetic field, magnetic field, and electric field:

“Light—an electromagnetic field of certain wavelength range—is a stream of photons; those of light are ‘on the mass shell’, those of electric and magnetic fields are not.”
— Martinus Veltman (2003), Facts and Mysteries in Elementary Particle Physics [11]

It is important to distinguish between on-shell and off-shell, in QFT, because on-shell and off-shell, according to Lubos Motl (2013), are opposite to each other, in a sense. On-shell describes fields that obey the equations of motion and real particles; off-shell describes fields that don't have to obey the equations of motion and are virtual particles. (Ѻ)

In modern physics parlance, particles are said to be “on the mass shell,” or simply “on-shell,” if their behaviour satisfies the relationship between energy and momentum given by Einstein in his theory of special relativity. Virtual particles are those that don’t satisfy this relationship, and are said to be “off-shell.” (Ѻ)

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.”

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Etymology
In 1917, American psychologist Leonard Troland (1889-1932) introduced the term "photon" in his article “The Measurement of Visual Stimulation Intensities”, a term usage which in 1922 he summarized as follows: [8]

“In order to facilitate the specification of the visual stimulus intensity in terms which shall be significant for the retinal image, the writer has suggested the employment of a special intensity unit called the photon. The photon is defined as that intensity of stimulation which accompanies the use of a pupillary area of one spare millimeter an external stimulus surface brightness of one candle per square meter.”

The use of the term photon here by Troland, however, has no quantum mechanical connection.

In 1923, American physicist Arthur Compton published his “A Quantum Theory of the Scattering of X-Rays by Light Elements”, wherein he gave experimental evidence for the particle view of light, in which he employed the terms: “light element”, “quanta of radiation”, “radiation quantum”, but not, it seems, the term photon. [9] Many, such as American science historian Isaac Asimov, claim that Compton coined the term photon in 1923. [4] The actual coining passage, however, remains to be tracked down. In any event, by 1924 American physicist Robert Millikan, in his The Electron, was summarizing things as such: [10]

“The next mode of interaction of light with electrons was discovered and explained in terms of a famous billiard-ball model proposed by Arthur H. Compton in 1923. Compton pictured a free, stationary electron as being hit by a ‘photon’ of light.”

In 1926, American physical chemist Gilbert Lewis, in his “The Conservation of Photons”, 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.”

Dozens, in fact the majority, of references misattribute the coining of the term photon to this 1926 article by Lewis, whereas correctly the term was in use for the particle of light by Millikan in 1924, if not by Compton the year prior. In 1935, Compton, in his discussion of the double slit experiment and the Heisenberg uncertainty relation, stated: [7]

“Recent experiments have shown that a light beam consists of a stream of particles—photons, we call them.”

In other publication, herein about this period, Compton also referred to photons as "radiation particles".

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.
7. Compton, Arthur H. (1935). The Freedom of Man (pg. 6). Yale University Press.
8. (a) Troland, Leonard T. (1917). “The Measurement of Visual Stimulation Intensities” (photon, 8+ pgs), Journal of Experimental Psychology, 2:24-32.
(b) Troland, Leonard T. (1922). “The Present Status of Visual Science” (photon, pg. 42), Bulletin of the National Research Council, Issues 26-31.
9. Compton, Arthur. (1923). “A Quantum Theory of the Scattering of X-Rays by Light Elements”, Physical Review, 21:483-502.
10. Millikan, Robert. (1924). The Electron (photon, pg. xxii-iii). University Press.
11. Veltman, Martinus. (2003). Facts and Mysteries in Elementary Particle Physics (pg. 17). World Scientific.

External links
● Photon – Wikipedia.