Recent results on the reflection of waves from ridged mirrors are discussed. Numerical calculations, analytical estimates and the direct measurements of coefficient of specular reflection of atomic waves from solid-state mirrors are combined. The reflectivity is approximated as an elementary function of period L of ridges, their width ell, wavenumber K, grazing angle θ and effective depth w of the van der Waals potential. In a special case L = ell, the fit reproduces the reflectivity of flat surfaces. Our approximation allows us to optimize the L at given ell and estimate the maximum performance of a ridged mirror. Such a mirror is suggested as a focusing element for the nano-scale imaging system.