Light enables us to see things. This tautology is the starting point of a long and fascinating trail from which developed both extraordinary views of how the natural world behaves and the very tools and methods on which modern science is built-explanations that are commensurate with experiments.

　　Light carries information about our surroundings, from distant stars and galaxies to the cells in our bodies to individual atoms and molecules. It is the basis of many technologies that enhance our quality of life: the Internet is powered by light; the most precise clocks in the world rely on light; the tiniest objects, from individual atoms, to live biological cells, can be observed and manipulated using light; images and displays are everywhere. Light reveals the full strangeness of the quantum world and inspires our imagination about the world. Yet it might surprise you to know that what light actually is was only really understood less than a hundred years ago, and even now we are ekeing out new insights from that understanding.

　　This book explores how people have come to our current view of what light is and what it does. It’s a great story, reaching back into the ancient world, with a global cast of contributors, from Euclid in Athens and Al-Hazen in Baghdad, conceiving the idea of light rays, to Ted Maiman in Los Angeles and Shuji Nakamura in Tokushima developing new lasers, by way of Joseph Fresnel in Paris and Thomas Young in London thinking about light waves, to James Clerk Maxwell in Aberdeen and Heinrich Hertz in Berlin developing from these ideas the concept of electromagnetic fields, that finally culminated in Albert Einstein in Bern and Paul Dirac in Cambridge explaining these apparently mutually incompatible facts by a radical new concept-the quantum field-that showed how it is possible to be both a particle and a wave at the same time.

　　At each step, new understanding (for example, of refraction) has led to new applications (such as eyeglasses for correcting vision): the path from discovery to technology is often very short when light is involved. The impact of light on the modern world is immense, and often unrealized. For this reason, 2015 has been designated by the United Nations as the International Year of Light: a celebration of light and what is made possible by it.

　　I should like to record my thanks to Alex Walmsley and Latha Menon for helpful comments on drafts of this book, and to many colleagues for responding to specific technical questions as well as those who kindly provided illustrations for the figures. Errors and statements that go beyond the exhortation, attributed to Einstein, to make things as simple as possible, but not simpler, are due to me.

　　INTERNATIONAL YEAR OF LIGHT 2015