Rami Arieli: " The Laser Adventure" Chapter 2.1 page 1
2.1 Bohr model of the atom.
Lasing action is a process that occurs in matter.
Since matter is composed of atoms, we need to understand (a little) about the structure of the atom, and its energy states.

We shall start with the semi-classical model, as suggested in 1913 by Niels Bohr, and called: The Bohr model of the atom.

According to this model, every atom is composed of a very massive nucleus with a positive electric charge (Ze), around it electrons are moving in specific paths.

Figure 2.1 illustrates a simple, but adequate, picture of the atom, the Bohr model:

Fig 2-1:Bohr picture of the Atom

Every "allowed orbit" of the electron around the nucleus, is connected to a specific energy level.
The energy level is higher as the distance of the "orbit" from the nucleus increases. Since for each atom there are only certain "allowed orbits", only certain discrete energy levels exist, and are named: E1, E2, E3, etc.