  What is light and how to describe it has always been a central subject in
physics. As our understanding has increased, so have our theories changed:
Geometrical optics, wave optics and quantum optics are increasingly
sophisticated descriptions, each referring to a larger class of phenomena than
its predecessor. But how exactly are these theories related? How and when wave
optics reduces to geometric optics is a rather simple problem. Similarly, how
quantum optics reduces to wave optics has been considered to be a very simple
business as well. It's not so. As we show here the classical limit of quantum
optics is a far more complicated issue; it is in fact dramatically more
involved and it requires a complete revision of all our intuitions. The revised
intuitions can then serve as a guide to finding novel quantum effects.
