Electromagnetism
A photon is an elementary particle, meaning that it cannot be broken down into smaller particles. It is the particle that light is made up of. Photons also make up all other types of electromagnetic radiation such as gamma rays, X-rays, and UV rays. The idea of photons was thought up by Einstein. Using his theory for the photoelectric effect, Einstein said that light existed in small "packets" or parcels which he called photons.