New insight into the classical Macdonald physical optics approximation

The Physical Optics approximation is widely used in analysis of antennas and scattering problems for electromagnetic and acoustic waves. The present paper investigates the nature of this classical approximation. It is shown that the scattered field in this approximation can be separated into two parts: the reflected field, containing all reflected rays and beams, and the shadow radiation, responsible for the Fresnel diffraction and the forward scattering. This observation elucidates the physics behind the fundamental diffraction law related to the total power scattered by large reflecting objects. It also clarifies the diffraction limit for reduction of scattering by absorbing materials.