Scattering of Electromagnetic Waves: Blue Skies, Cerebral Oximetry, and Some Reassurance About X-Rays

Abstract

The trajectory of an electromagnetic wave which travels through a transparent medium may be deviated in any direction as a result of the interaction between the wave and some obstacle or “inhomogeneity,” such as the particles that are suspended in the medium. This phenomenon, which is referred to as scattering of the electromagnetic wave, explains why the sky is blue. While nonionizing electromagnetic waves such as visible light and infrared radiation mainly undergo elastic scattering (in which the wavelength of the scattered radiation does not change), scattered ionizing radiations undergo a change in wavelength (inelastic scattering). Both types of scattering may have a role in the daily practice of anesthesia and intensive care medicine. For example, the elastic scattering of infrared radiation is one of the principles on which near-infrared spectroscopy (NIRS) cerebral oximetry is based and, at the same time, represents a major technological challenge which accounts for some limitations of this technique. Moreover, the special kind of (inelastic) scattering which ionizing radiations undergo (Compton scattering) has a key role in radiation protection. Finally, although nonionizing electromagnetic waves mainly undergo elastic scattering, inelastic scattering may also occur: this phenomenon, which is known as Raman scattering, is used in some types of respiratory gas analyzers.