The Photon
A photon is an elementary particle that serves as the quantum of the electromagnetic field, including light and radio waves. It is the force carrier for the electromagnetic force. Unlike many other particles, the photon has zero invariant mass, allowing it to travel at the speed of light in a vacuum.
The modern concept of the photon was developed gradually by Albert Einstein in the early 20th century to explain experimental observations that did not fit the classical wave model of light. In particular, his explanation of the photoelectric effect, for which he received the Nobel Prize in Physics, demonstrated that light energy is quantized. This work built upon the earlier research of Max Planck, who introduced the idea of energy quanta.
According to Quantum Mechanics, photons exhibit wave-particle duality, meaning they display properties of both waves and particles. This means they can be described as both a discrete packet of energy and a continuous wave. For further reading, see the Encyclopædia Britannica or the official CERN documentation on the Standard Model.