The electromagnetic (EM) fi eld is a physical infl uence (a fi eld) that permeates through all of space. It arises from electrically charged objects and describes one of the four fundamental forces of nature, electromagnetism, which is found almost everywhere. All EM fi elds are force fi elds, carrying energy and capable of producing an action at a distance. These fi elds have characteristics of both waves and particles. This energy is utilized in various ways, though we still lack a full understanding of its fundamental properties. Many inventions of the late twentieth century, ranging from everyday home and offi ce appliances to satellite systems and mobile phones, are so important and so advantageous, we wonder how we ever lived without them. Table 1.1 shows a few examples of EM sources.