A Photon is one of the basic structures of the universe. In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. As an elementary particle, it acts as one of the fundamental forces within the electromagnetic field and is considered in the discipline of a particle physics to be the basic unit of light (in the known Universe, you can't get light with in smaller units than this). At both microscopic and macroscopic level, the effects of electromagnetic force as caused by photons can be readily observed in the interactions of the physical world. Like all elementary particles, photons are currently best explained by Quantum mechanics and will exhibit wave-particle duality, exhibiting properties of both waves and particles.
Photons were first found by Max Planck in 1900 as “packets” of energy, that he referred to as “Quanta”. This was followed by research conducted by Albert Einstein, who identified these packets as electromagnetic waves in 1905. The term photon was coined by Gilbert Lewis in 1926, though the concept of light in the form of discrete particles had been around centuries and had been formalized in Newton’s construction of the science of optics. In 1800’s however, the wave properties of light became slightly obvious and scientists had essentially thrown the particle theory of light out of the window until Albert Einstein explained the “photoelectric effect”.
The photoelectric effect or the Hertz effect, is the process by which metal emit electrons as a result of photons impacting on the metal plates. Photoelectric effect lead to our understanding of the dual nature of light - as waves and particles (photons). In a light beam, photons have a defined energy based on the frequency of the light. What the researchers observed was that the photoelectric effect came into play and electrons were emitted by the metals, when the light was of a certain frequency that is above the threshold frequency for that metal. The metals would not emit any electrons, even if the intensity of the light was significantly increased, while holding the frequency of the light below the threshold frequency. This lead them to conclude that increasing the intensity of the light increased the number of photons in the beam, and hence increased the number of electrons excited in the metal, but did not increase the energy transferred by the photons to the electrons. They concluded that the energy of the photon is determined by the frequency of the light.
If light were to be only "waves", then increasing the intensity of the light alone should have resulted in more energy being transferred to the electrons, and hence the researchers should have detected emitted electrons. But that was not what they observed. Even at very low intensities, researchers observed electron photoemissions as long as the frequency of the light beam was above the threshold frequency of the metal. When the frequency is above the threshold frequency, number of electrons emitted would go up with increased intensity as more photons will be impacting the atoms and exciting the electrons. This lead them to conclude that light was in fact made of particles (when interacting at the sub-atomic level) and behaves like a wave at the macro-level (e.g., interference, diffraction around objects)
Some properties exist with photons that make them unique as compared to other elementary particles. First, a photon is mass less. It has no electric charge. It does not decay spontaneously in empty space since it has no smaller sub particles. When a charge, either positive or negative is accelerated near the speed of light, synchrotron radiation is created, which causes the photons to be released. In addition to that, photons can be emitted when the energy of molecules, atoms, nuclei alter to a lower level. Based on Quantum physics, when electron-positron (an antimatter equivalent to electron) eradication occurs, meaning a particle and anti particle is removed; photon light is also emitted.
It is almost certainly impossible to do any kind of experiment which would establish that the photon rest mass is exactly zero. The best is to place limits on it. A non-zero rest mass would lead to a change in the inverse square Coulomb’s law (law describing the electrostatic interaction between the electrically charged particles) of electrostatic forces. There would be a small damping factor making it weaker over very large distances. The behavior of static magnetic fields is likewise modified. A limit on the photon mass can be obtained through satellite measurements of planetary magnetic fields. If a photon is a mass less particle, it would not move at the exact speed of light in vacuum, C. Rather, its speed would be lower and dependent on its frequency. Relativity would be unaffected by this; the so called speed of light, c, would then not be actual speed at which the light moves, but a constant of nature which is the maximum speed that any object could theoretically attain space-time. Therefore, it would be the speed of space time ripples (gravitational waves), but it would not be the speed of photons.
Photons have a number of applications in industry and technology.