The restrictions on the applicability of ray optics in the turbulent atmosphere are investigated. Within the single-scatter approximation, the requirement that amplitude fluctuations have negligible effect compared to phase fluctuations leads to the condition linner11/3louter1/3/λ2>>(L/27)2 where linner, louter are the inner and outer turbulence scales and L is the range. This criterion is a considerable relaxation of the criterion which has been employed (linner2/λ>>L).
LEONARD S. TAYLOR, "Validity of Ray-Optics Calculations in a Turbulent Atmosphere," J. Opt. Soc. Am. 58, 57-59 (1968)