- #1

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My problem is with the equation. I know that they are considering the loop only, leaving out the external Fermion and photon lines. I understand how the two propagators, ##iS_F(p' + k)## and ##iS_F(p + k)## have come, and also how the last term has come (from the propagator of photon field). But why are the ##ie\gamma## present before each propagator term? While writing the Feynman amplitude, we don't add these terms. Why are we adding them here?

*N.B.: Sorry for not typing out the equation. It was a long one, and I thought you can understand from the scan itself, so I posted a screenshot. In case of any discrepancy, let me know, and I shall type it out.*