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. The two pillars of this framework, quantum mechanics and relativity, together with the ideas of statistical physics that allow us to apply quantum mechanics to macroscopic objects (such as hot coals), 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 a few experiments of our own in class. We will discuss the interaction between experiment and theory, as well as the roles of simplicity, elegance and unity in the search for explanations. This is a small seminar designed for first-year students who have placed out of Physics 141. Teams of students will be responsible for leading some of the class sessions, and at the end of the semester each student will give a short oral presentation. Format: lecture and discussion three hours a week; occasional lab experiments done in class. Evaluation is based on class participation, weekly problem sets, an oral presentation, two hour-exams and a final exam, all of which have a substantial quantitative component. Prerequisite: placement by the department (see advanced placement above). Students may take either Physics 142 or Physics 151 but not both. Enrollment limit: 20 (expected: 14). This is a quantitative/formal reasoning course.