  If a time-dependent Hamiltonian is allowed to evolve adiabatically, and if it
returns to its original form, then the ground state wavefunction must have
picked up the dynamic or(and) the geometric phase factor(s) due to some
interaction during the above evolution. Here, (i) we invoke the Pancharatnam's
notion of phase retardation to show that the microscopic origin of Berry's
phase is due to a change in the phase velocity (of a wavefunction), and (ii)
the dynamic phase factor is shown to have its origin in the group velocity of a
wavefunction. We also expose that the emitted photon's polarization during an
electronic transition has its origin in the changes of the phase velocity of
the electron wavefunction. Here, we prove the above statements, (i) and (ii),
such that they are not due to some interpretational issues, hence, an
expos${\rm \acute{e}}$.

General Physics