What is the fundamental particle of light?

The fundamental particle of light is the photon. A photon is a single quantum, or indivisible packet, of the electromagnetic field. Visible light consists of photons, but photons also carry other forms of electromagnetic radiation, including radio waves, infrared, ultraviolet, X-rays, and gamma rays.

The modern idea developed from the work of Max Planck and Albert Einstein. In 1905, Einstein argued that electromagnetic radiation sometimes behaves as though it is made of discrete energy packets, helping explain the photoelectric effect. The name photon was later popularized by physicist Gilbert N. Lewis in 1926.

Photons have zero electric charge and zero rest mass. In a vacuum, every photon travels at the speed of light, although its energy depends on its frequency: higher-frequency photons carry more energy. Photons also show wave-particle duality, behaving like waves in interference experiments and like particles when detected individually.

A common mix-up is to call an electron or proton a particle of light. Those are matter particles; a photon is the massless force carrier of the electromagnetic interaction.