Rami Arieli: "The
Laser Adventure" Chapter 2.2 page 2
Summary
The interactions between electromagnetic radiation and
matter cause changes in the energy states of the
electrons in matter.
Electrons can be transferred from one energy level to
another, while absorbing or emitting a certain amount
of energy. This amount of energy is equal to the energy
difference between these two energy levels (E2-E1).
We shall see later in this chapter that:
When this energy is absorbed or emitted in a form of electromagnetic
radiation, the energy difference between these two energy levels
(E2-E1) determines uniquely the frequency (n)
of the electromagnetic radiation: