Figure 19: Instantaneous wave fronts in
the spacetime of a transparent Barriola-Vilenkin monopole
with . In addition to the tangential caustic of
Figure 18, a radial caustic is formed. For each instantaneous wave front, the radial caustic
is a cusp ridge. The radial caustic of the entire light cone is a lightlike 2-surface in spacetime
which projects to a rotationally symmetric 2-surface in
3-space.
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