Here are the slides for an undergraduate lecture I gave today. The slides also include the derivations I gave by hand on the document viewer.For historical background, I found a book chapter from Out of the Crystal Maze, interesting and helpful.Some fascinating context concerning Arnold Sommerfeld. Over a period of 32 years, he built up an amazing school of theoretical physics in Munich. He mentored seven future Nobel laureates. However, sadly, when he retired in 1934 from a Chair in Theoretical Physics, it took four years to replace him because of interference from the Nazi Party. Even worse, in the...