Soft photon

In particle physics, soft photons are photons with energies much smaller than the energies of the particles participating in a particular scattering process, and they are not energetic enough to be detected. Such photons can be emitted from (or absorbed by) the external (incoming and outgoing) lines of charged particles of the Feynman diagram for the process. Even though soft photons are not detected, the possibility of their emission must be taken into account in the calculation of the scattering amplitude.