Teaches the often counterintuitive physics of quantum mechanics by working through detailed applications of general ideas. This book uses the hyperfine structure of atomic hydrogen (the 21 cm line): the computation of the energy splitting and the induced and spontaneous transition rates. It emphasizes the art of numerical estimates. "Peebles applies quantum theory, often in simple, approximate ways, to a variety of interesting problems . . . [It] could prove quite a rewarding book for the more able and motivated student."