Rainbow
A virtual photon emitted and reabsorbed by the same fermion line, arcing over a segment of the propagator like a rainbow. The simplest self-energy correction for a fermion.
Structure
Two vertices on a single fermion line, connected by a photon propagator that bypasses the fermion segment between them. The fermion propagates between the two vertices with modified momentum.
Where it appears
Electron self-energy: the electron emits and reabsorbs a virtual photon. UV and IR divergent. Generates mass renormalization and contributes to the Lamb shift.