Principle of Laser
In laser, the photons interact with the atom as absorption of radiation, stimulated emission, and spontaneous emission. To understand the principle of laser one should know these principles.
Absorption of Radiation
Absorption of radiation is the process by which electrons in the ground state absorbs energy from photons to jump into the higher energy level.
Spontaneous emission is the process by which electrons in the excited state return to the ground state by emitting photons. As per quantum mechanics, the lower energy level is more stable than higher energy levels. Due to this, the electrons tend to occupy the lower level. The electrons in higher energy levels decay into lower levels, with the emission of EM radiation.
Stimulated emission is the process by which an incident photon interacts with the excited electron and forces it to return to the ground state.