Gain at a Point and Gain in a Direction for the Fraunhofer Diffraction Pattern of a System of Many Wave Sources

We discuss the question of the nature of the amplification in the direction of wave motion generated by a system of point coherently emitting sources. It is proved that in the Fraunhofer picture this amplification does not have a point nature but produces at all points of the frontal part of the observation area, and by the same amount. The conditions are found under which the wave field within the area perpendicular to the direction of observation and commensurate with the transverse dimensions of the source region can be approximated by a plane wave. It is shown that in some cases it is necessary to have a collecting lens to obtain a local maximum (maximum at a point) in diffraction experiments.