It's almost always the last topic in the first semester of introductory physics—angular momentum. Best for last, or something? I've used this concept to describe everything from fidget spinners to ...

Figure 1: The wave front (top) and the intensity pattern (bottom) of the simplest Laguerre–gaussian (LG l p) or ‘doughnut’ mode. It is only by using a coincidence measurement that we could show that ...