This book is concerned with the interaction of people and light. To fully understand this interaction, it is first necessary to understand what light is, how its characteristics can be quantified and how it is produced and controlled. These topics are the subject of this chapter. To the physicist, light is simply part of the electromagnetic spectrum that stretches from cosmic rays with wavelengths of the order of femtometres to radio waves with wavelengths of the order of kilometres (Figure 1.1).