Answers to your questions: 1): all the way to the left, a mirror with a reflectivity|r| of 1 (or a 100%). In the middle an |r| of slightly below 1. Yes, optical, system with photons (a and a^dagger with [a,a^dagger]=1). 2) distance between mirrors 1 and 2: l. Distance mirror 2 and 3:L. (Later taking the limit L/l ==>> infinity) 3) the how is actually correct, I guess the word behaves is missing twice: .... how .... behaves and a .... behaves.