  The nature of changes in the interference pattern caused by the presence of
polarization-changing elements in one or both beams of an interferometer, in
particular those caused by an effective optical activity due to passage of a
polarized beam through a coiled optical fiber are clarified. It is pointed out
that for an incident state that is not circularly polarized so that the two
interfering beams go to different polarization states, there is an observable
nonzero Pancharatnam phase shift between them which depends on the incident
polarization state and on the solid angle subtended by the track of the
$\vec{k}$-vector at the centre of the sphere of k-vectors. The behaviour of
this phase shift is singular when the two interfering states are nearly
orthogonal. It is shown that for zero path difference between the two beams,
the amplitude of intensity modulation as a function of optical activity is
independent of the incident polarization state.

Optics;General Physics