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hf =E2- El The left hand side of this equation describes the quantum of energy associated with a photon of light while the right hand side describes the change undergone by the atom to produce this photon of light.

hf 20



hf 10



I Eo




Figure 1 The allowed energy transitions from the first excited level of a threelevel atom: 1) Spontaneous emission wherein the excited atom gives up energy of excitation in the form of a light photon; 2) Absorption of energy in the form of a photon of the energy radiation in the environ­ ment of the atom; 3) Stimulated emission induced by an environmental photon.

Let us represent the allowed energy states in an idealized atom as in Figure 1. Einstein described three ways in which an atom in the energy state Eq can leave the state. First, if there is no external source of energy available to the atom it cannot move from state Eq to a higher state E2. However, there is a finite probability that the atom will spontaneously {i.e. without 29

Otterbein Miscellany May 1965 combined  
Otterbein Miscellany May 1965 combined