The photon is an elementary particle and the fundamental quantum of electromagnetic radiation. It does not consist of smaller particles but is considered a fundamental particle itself.
In the Standard Model of particle physics, the photon is classified as a gauge boson, specifically the gauge boson associated with the electromagnetic force. Gauge bosons are particles that mediate the fundamental forces in nature. For example, the photon mediates the electromagnetic force, while other gauge bosons, like the W and Z bosons, mediate the weak nuclear force.
The photon is massless, meaning it has zero rest mass. It carries a unit of electromagnetic energy and momentum and interacts with charged particles. It has spin 1 and is described as a massless, spin-1 particle with two possible polarization states.
The properties and behavior of the photon are described by quantum electrodynamics (QED), which is the quantum field theory that describes the electromagnetic interaction. In QED, the photon is associated with the quantized electromagnetic field, and its interactions with charged particles are described through Feynman diagrams and calculations involving the electromagnetic coupling constant (fine-structure constant).
In summary, the photon is an elementary particle and the fundamental quantum of electromagnetic radiation. It is not composed of smaller particles but is considered a fundamental particle itself, described by quantum electrodynamics.