Multi-plane lensing in wave optics
  Wave effects in lensing form a rich field on the intersection of classical
caustic singularities and quantum interference, yet are notoriously difficult
to model. I present a new method to define and efficiently evaluate multi-plane
lensing of coherent electromagnetic waves by plasmas and gravitational lenses
in polynomial time. The large number of recently observed pulsars and fast
radio bursts in radio astronomy suggest that wave effects will likely be
observed in the near future. The interference fringes are sensitive to physical
parameters which cannot be inferred from geometric optics. In particular, for
multi-plane lensing, the pattern depends on the redshifts of the lens planes.
The study of these effects will allow the use of radio sources to probe our
universe in novel ways.
