About the Book: This book, a continuation of Introduction to Modern Physics: Theoretical Foundations (Vol. I), deepens students’ understanding of twentieth-century theoretical physics. It covers quantum mechanics reformulation, angular momentum, scattering theory, Lagrangian field theory, symmetries, Feynman rules, quantum electrodynamics, path integrals, and canonical transformations, with problems and appendices providing a self-contained, in-depth learning framework.