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Albert Einstein predicted that light exists in tiny particles in 1905 and they are now known as photons. The idea was that light was quantized and broken into discrete packets or quanta. Each of these packets of light had their own wavelength, energy and frequency. This was the explain a phenomenon known as the photoelectric effect: a beam of light will hit a metal surface and electrons will be released. This was one of Einstein's great papers of 1905 and was building off of Max Planck's work 5 years earlier on the quantization of the full spectrum of thermal radiation and led to the development of quantum mechanics in the subsequent decades.
The photon is the quanta of all forms of electromagnetic radiation, including light. It is the force mediating particle for electromagnetism and is described by a theory called quantum electrodynamics.
Atoms can emit photons of light. In the model of the atom proposed by Niels Bohr in 1913, the energy of an electron is determined by the distance that it's orbital shell is from the atomic nucleus. When an electron jumps through empty space to a lower energy shell below it the electron falls into that lower energy shell. Thus, the energy that the electron lost is emitted as a photon or particle of light.
The experimental evidence for photons came with the experiment of Arthur Compton in 1923. Compton demonstrated that photons have a kind of momentum. A beam of X-rays were shot at a block of graphite at a specific frequency. The scattered radiation had a lower frequency. Compton explained the drop in frequency to be related to the fact that light is made of particles. Photons, also, cannot have any mass. This is because they travel at the speed of light and have a finite energy content. This is a consequence of special relativity: anything with mass cannot reach light speed.
A Feynman diagram is a pictorial representation of the interactions of subatomic particles and were first proposed in 1948 by Richard Feynman. In a Feynman diagram, fermions are straight lines and bosons are represented as wavy lines. Time will be represented by one axis and space as another. A Feynman diagram can visually represent how two approaching electrons can exchange a photon and then move apart from one another. It can also represent how an electron and an antielectron (positron) can annihilate and produce a photon in the process. The particles will continue to move away, however, as a muon and an antimuon.
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