  In this note I introduce a mysterious approximation called the rotating wave
approximation (RWA) to undergraduates or non-experts who are interested in both
Mathematics and Quantum Optics. In Quantum Optics it plays a very important
role in order to obtain an analytic approximate solution of some
Schr{\"o}dinger equation, while it is curious from the mathematical point of
view. I explain it carefully with two coherent oscillations for them and expect
that they will overcome the problem in the near future.
