This book bridges a gap between introductory optics texts and the vanguard of optical science, where light is used as a tool to probe the properties of new materials. A single mathematical tool is introduced that enables readers to understand laser tweezers, laser cooling, optical magnetism, squeezed light, and many other advanced topics. It is a significant advantage that the author has written a text that can be used by practitioners in the field with a wide variety of backgrounds, from conventional physics to bio-physicists and photo-chemists.