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Photons are energy packets of electromagnetic radiation, usually given the symbol γ (the third Greek letter, gamma). Light consists of photons. They can be produced in a variety of ways, including emission from electrons as they change energy states or orbitals. Photons can also be created by nuclear transitions, particle-antiparticle annihilation or any fluctuations in an electromagnetic field which gives rise to electromagnetic radiation.
In a vacuum, photons move at the speed of light c, around 3x10 meters per second. Light of frequency f consists of photons of energy E = h f and momentum p = h f / c, where h is Planck's constant (see wave-particle duality). As photons move through media, they will slow down at rates dependent on their frequency, refracting the light, and causing light of different frequencies and colours to separate into a spectrum.
Photons are believed to be fundamental particles, have a definite finite energy at the speed of light, but zero invariant mass. Even so, the theory of general relativity states that they are affected by gravity, and this is confirmed by observation.
see also particle physics, photonics, optics and spectroscopy