For xray diffraction measurements using a DebyeScherrer powd

For x-ray diffraction measurements using a Debye-Scherrer powder camera, if the beam wavelength is greater than twice the plane spacing there is a problem with observing any beam diffracted from those planes. Why?

Solution

Each diffraction line is made by the large number of spots which each spot emerges from seperate crystals. These spots are so small and very difficult to identify. These appears to be continous line. If the crystal is not grounded finely these spots appears to be sprinkle and very hard to distinguish the beam of diffractions.