Why does a hot coal glow red rather than blue or green or some other color? Remarkably, this simple question could not be answered before the year 1900, because the answer depends on a radical assumption introduced in that year by Max Planck. His work on thermal radiation marked the beginning of a revolutionary era in the history of physics that culminated in a new framework for our understanding of the physical world. Relativity, quantum mechanics, and statistical physics are the pillars of the modern framework, and constitute the core of this course. As we study this material, we will also be exploring the process of research in physics, partly by doing some experiments of our own. We will discuss the interaction between experiment and theory, as well as the roles of simplicity, elegance, and unity in the search for explanations.