  We theoretically describe the quasi one-dimensional transverse spreading of a
light pulse propagating in a nonlinear optical material in the presence of a
uniform background light intensity. For short propagation distances the pulse
can be described within a nondispersive approximation by means of Riemann's
approach. For larger distances, wave breaking occurs, leading to the formation
of dispersive shocks at both ends of the pulse. We describe this phenomenon
within Whitham modulation theory, which yields an excellent agreement with
numerical simulations. Our analytic approach makes it possible to extract the
leading asymptotic behavior of the parameters of the shock.
