Some Aspects of Diffraction Theory and their Application to the Ionosphere

In Part I the `Fourier' approach to diffraction theory is outlined in such a way as to emphasize its physical significance. Particular attention is paid to diffraction phenomena of the Fresnel type, and to diffraction from screens which alter both the phase and the amplitude of the incident wave. The `Fourier' approach is extended to deal with problems of `random' diffraction, for which purpose the use of auto-correlation functions is introduced and explained in physical terms.