In a vacuum, photons move at the speed of light *c*, around 3x10^{8} 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 dependant 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 particle physics, optics and spectroscopy