<Tr> <Th> C parity </Th> <Td> − 1 </Td> </Tr> <Tr> <Th> Condensed </Th> <Td> I (J) = 0, 1 (1) </Td> </Tr> <P> A photon is a type of elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual particles). The photon has zero rest mass and always moves at the speed of light within a vacuum . </P> <P> Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave--particle duality, exhibiting properties of both waves and particles . For example, a single photon may be refracted by a lens and exhibit wave interference with itself, and it can behave as a particle with definite and finite measurable position or momentum, though not both at the same time . The photon's wave and quanta qualities are two observable aspects of a single phenomenon, and cannot be described by any mechanical model; a representation of this dual property of light that assumes certain points on the wavefront to be the seat of the energy is not possible . The quanta in a light wave cannot be spatially localized . </P>

A packet or unit of light energy is called a