Diffraction will be best if the gap is equal to the wave length, but what <br>will be if the gap is smaller than the wave length? <br>
When gap is smaller than the wave length only main maximumwith zero order can occur. After passing grating flat wave remains flat, another beams do not occur. Grating behaves like plane-parallel plate. But that is true only in the distance of the grating. There exists non-uniform waves in thin layer near grating's surface. This waves quickly attenuate away from grating.