Textbook in PDF format This book explains in an accessible way how the concept of the photon was introduced by Einstein who also helped discover many of its properties, and how it interacted with many physical phenomena while revolutionizing the world of physics. Replete with short biographies of the main contributors to the history, the book begins with the historical and scientific conditions that led to the quantization of the electromagnetic field. This upended previously held assumptions about the nature of radiation and led to the christening of the photon