If the atom is in the excited state E2 and a photon of energy exactly equal to E2 – E1 = hυ is incident on it, then the incident photon interacts with the atom in the excited state and then it stimulates or induces the atom to come down to the ground state E1. A fresh photon is emitted in this process. Therefore, when an atom ejects a photon due to its interaction with a photon incident on it, the process is called stimulated emission (or induced emission).
Characteristics of stimulated emission:
a) For each incident photon, there are two outgoing photons moving in the same direction.
b) As the emitted photon has exactly the same energy, phase and direction as the incident photon, we will achieve an amplified and unidirectional coherent beam.
Note: The laser is based on the principle of stimulated emission.