Displacement of rays in a turbulent medium

The Fokker-Planck equation associated with the fluctuation (ray) equation is derived. The resulting equation for an isotropic turbulence is in the form of a diffusion equation. It has been found that the diffusion coefficient is more naturally defined in terms of the logarithmic refractive index. In the limit of weak turbulence, the present definition reduces to the conventional one. For a general and arbitrary "initial" condition, certain moments of the diffusion equation have been obtained. These moments give 1) the probability of finding a ray with a given ray direction and given path length, and 2) the mean and mean-square position of the ray in a given direction and given path length.