Scheme of typical effects experienced by the electromagnetic wave propagating through a slab of randomly inhomogeneous medium of thickness s. The conventional phase screen approach simulates the random corrugation of the incoming wavefront (a). The phase derivative screen adds the random refraction of the incoming signal ray (b). In the case of a sufficiently strong refractive index gradient ∇δn, the ray bends within the layer and appears to be displaced at distance δr₁ from the original direction of propagation ℓ₀. The dashed vector shows that, in contrast to case (a), a different ray emerges in the initial direction of ray propagation. The fluctuating part of the gained phase along the initial direction is then given by equation (14).