Before revision: The best way to understand linear optics is by comparing it to nonlinear optics: In linear optics (from quantum optics point of view), the unitary transformation on a single photon strictly determines the unitary transformation on any number of photons, while in nonlinear optics it does no not.
After revision: The best way to understand linear optics is by comparing it to nlinear optics: In linear optics (from quantum optics point of view), the unitary transformation on a single photon strictly determines the unitary transformation on any number of photons, while in nonlinear optics it does no not.