where ℓ ℓ is width of the ridges, L is distance between ridges, θ θ is grazing angle, and K = m V / ℏ ℏ is wavenumber and r 0 ( C , k ) is coefficient of reflection of atoms with wavenumber k from a flat surface at the normal incidence. Such estimate predicts the enhancement of the reflectivity at the increase of period L ; this estimate is valid at K L θ θ 2 ≪ ≪ 1 . See quantum reflection for the approximation (fit) of the function r 0 .