text: For narrow ridges with large period , the ridges just blocks the part of the wavefront. Then, it can be interpreted in terms of the Fresnel diffraction of the de Broglie wave, or the Zeno effect; such interpretation leads to the estimate the reflectivity
   is wavenumber,  is period (distance between ridges) and
  where the grazing angle  is supposed to be small. This estimate predicts enhancement of the reflectivity at the reduction of period . This estimate requires that .