Introduction

We live in an information age that is built almost entirely on the physics of electromagnetic (EM) waves. Television, cellphones and the internet all use EM waves as carriers to move signals from one place to another, and even the energy that sustains life on Earth arrives from the Sun as EM waves that cross roughly 150 million km of empty space before reaching us. EM waves are emitted by an enormous range of sources besides the Sun -- ordinary light bulbs, the heated engine blocks of automobiles, X-ray machines, lightning flashes, radioactive materials, and stars and galaxies far beyond our own. Because EM waves are so central to both nature and technology, this chapter makes a careful study of their properties, starting from the fundamental physics that predicts their existence.

The chapter has two connected halves. The first half (sections 13.2 to 13.4) develops the physics of EM waves themselves: the laws that predict them, their characteristic properties, the electromagnetic spectrum they span, and how they propagate from a transmitter to a receiver through the Earth's atmosphere. The second half (sections 13.5 and 13.6) builds on that physics to introduce communication systems -- the technology that deliberately uses EM waves (mainly radio waves) to carry information such as speech, music and video over long distances, culminating in the idea of modulation, which is what makes long-distance wireless communication practically possible at all.

Before starting, it helps to recall a few ideas from earlier physics: what a wave is and the distinction between longitudinal and transverse waves; what electric and magnetic fields are and what sources produce them; Lenz's law, Ampere's law and Faraday's law, which describe how electric and magnetic fields relate to charges, currents and changing fluxes; and the mechanisms (such as radiation) by which hot bodies lose heat. All of these ideas reappear directly in the next section, where they are drawn together into the set of laws that predicts the existence of EM waves.