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.