A photon is the quantum of electromagnetic radiation, carrying energy proportional to its frequency.
The photon concept developed from Max Planck’s 1900 quantum hypothesis and Albert Einstein’s 1905 explanation of the photoelectric effect. A photon’s energy is given by E = hf, where h is Planck’s constant and f is the radiation frequency. Higher-frequency light therefore has more energetic photons.
Photons have zero rest mass and travel at the speed of light in vacuum. They carry momentum despite having no rest mass, with momentum related to their wavelength. Electromagnetic radiation across the spectrum—including radio waves, visible light, ultraviolet radiation, X-rays, and gamma rays—consists of photons.
A photon is different from a phonon, which is a quantized excitation of vibrations in a material. Photons can be absorbed or emitted by atoms, but their energy is not determined by brightness; brightness generally reflects the number of photons or the beam’s intensity.