  It is well known that in classical optics, the visibility of interference, in
a two-beam light interference, is related to the optical coherence of the two
beams. A wave-particle duality relation can be derived using this mutual
coherence. The issue of wave-particle duality in classical optics is analyzed
here, in the more general context of multipath interference. New definitions of
interference visibility and path distinguishability have been introduced, which
lead to a duality relation for multipath interference. The visibility is shown
to be related to a new multi-point optical coherence function.
