- Fundamental particle of electromagnetic radiation. Different problems require radiation to be thought of as a particle or as a wave, and the shorter the wavelength of the radiation the more energetic the equivalent photon.
- A particle with no mass nor charge, and which composes light and other forms of electromagnetic radiation. gamma rays and X-rays are examples of higher-energy photons, while infrared rays and radio waves are examples of lower-energy forms. Its energy is equal to hf, where h is the Planck constant, and f is the frequency of the radiation in Hz. Photons are required to explain phenomena such as the photoelectric effect, in which light has particle-like properties. The photon is the carrier of the electromagnetic force, which is the force responsible for all electromagnetic interactions, such as those which hold atoms and molecules together. Also called light quantum.