This textbook explains the basic principles and approximate methods used in quantum mechanics with special focus on the use of symmetries in solving practical problems. It demonstrates that symmetries offer "elegant” solutions for problems which can be solved accurately and significantly simplify solutions of problems whose solutions are only approximate.
It also provides, with the minimum necessary formalism, an introduction to quantum electrodynamics and its use in low energy physics. It systematically analyzes the influence of relativistic, magnetic and quantum electrodynamic effects on atomic spectra.