ABSTRACT
This chapter begins with an evaluation of the cross-sections of individual and correlated charged particles against scattering of electromagnetic waves. It looks at the radar backscattering experiments that demonstrate those collective processes in the ionosphere. X-ray Thomson scattering techniques have likewise been employed for observation of collective modes in warm dense matter. The chapter shows that the total cross-section of an electron against scattering of the electromagnetic wave is given by the cross-section of Thomson scattering. Generally, however, the frequency of the scattered wave differs from that of the incident wave due to the Doppler effect associated with the motion of electrons. It describes the possibility that the temperatures of the electrons and the ions may be different. The chapter argues that is identical in analytical form to the results obtained for the critical fluctuations in the vicinity of a liquid-gas phase transition, or the critical opalescence.
