Focusing on the unresolved debate between Newton and Huygens from 300 years ago, The Nature of Light: What is a Photon? discusses the reality behind enigmatic photons. It explores the fundamental issues pertaining to light that still exist today. Gathering contributions from globally recognized specialists in electrodynamics and quantum optics, the book begins by clearly presenting the mainstream view of the nature of light and photons. It then provides a new and challenging scientific epistemology that explains how to overcome the prevailing paradoxes and confusions arising from the accepted definition of a photon as a monochromatic Fourier mode of the vacuum. The book concludes with an array of experiments that demonstrate the innovative thinking needed to examine the wave-particle duality of photons. Looking at photons from both mainstream and out-of-box viewpoints, this volume is sure to inspire the next generation of quantum optics scientists and engineers to go beyond the Copenhagen interpretation and formulate new conceptual ideas about light--matter interactions and substantiate them through inventive applications.