Beam propagation in a Randomly Inhomogeneous Medium
  An integro-differential equation describing the angular distribution of beams is analyzed for a medium with random inhomogeneities. Beams are trapped because inhomogeneities give rise to wave localization at random locations and random times. The expressions obtained for the mean square deviation from the initial direction of beam propagation generalize the "3/2 law". 
Aleksander Stanislavsky
astro-ph.IM cond-mat.stat-mech