Photon

A photon is a quantum of electromagnetic wave energy. As a particle, a photon has zero rest mass, moving exclusively at the speed of light. As an elementary particle, a photon belongs to the bosons, i.e. it has an integer spin. Although it has zero rest mass, it has non-zero (relativistic) momentum, which manifests itself, for example, in collisions (Compton scattering, radiation pressure,...).

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