The diagram represents photons interacting with excited atoms in the active medium of a laser. How many photons will pass through the dashed region shown in the diagram? Assume that all atoms remain in excited states until they emit a photon.
In our diagram, we have three atoms here, here, and here. And besides these atoms, we see photons. For the atom on the left, we have a photon coming in and interacting with that atom, which then leads the atom to decay to a lower energy state and in the process emit a photon identical to the one originally interacting with it. We see then that by this atom decaying down to a lower energy state, it has doubled the number of photons. This process is repeated for both of the atoms on the right. Each one has a photon interact with it. And these photons are identical in frequency to our original interacting photon.
In each case therefore, we expect both of these atoms to drop down to a lower energy state and emit a photon that is identical to the interacting photon that stimulates this emission. Each of the atoms on the right then emits its own photon. We can say that these two are those, and these are copies of the two original interacting photons that, rather than being absorbed, still exist. Knowing all this lets us answer our question. The number of photons that pass through the dashed region shown in the diagram is four, one for each of the atoms shown on the right in our diagram and one for the stimulation of photon emission from each atom, giving a total of four.