A new approximation method for solving the problem of light scattering by optical inhomogeneities (modified approximation of abnormal diffraction) has been considered. A rigorous mathematical validation of the method is presented, the ways of its further correction (with allowance for the volume diffraction with the use of Rytov phase) are given, the possibility for rigorous formulation of the problem of determination of the field of applicability and the velocity of convergence of the approximate solution are validated. It is shown that the field of applicability of the approximation is substantially expanded as compared to the Hulst approximation. The new method is suitable for calculation of the scattering phase function at any values of the scattering angle including the backscattering.