A linear oscillatory system has a restoring force that varies linearly with position. However, in almost all natural oscillatory systems there is always some damping involved and thus the oscillatory system loses energy to friction or dispersion. The damping of oscillations resulting from friction can be counteracted if some mechanism supplies the system with energy from an external source at a rate equal to that absorbed by the damping medium. This counteracting mechanism is usually referred to as driven or forced oscillation.
Resonance occurs when the amplitude of a driven and damped oscillatory system is at a maximum.
This resonant frequency is a function of the driving frequency, w 0, and the damping coefficient, B, of the system. If only a very little damping occurs in the system then the Q value is very large and the amplitude of oscillation at resonance is illustrated by a sharply defined peak. If the damping is larger, then the Q value is much smaller so that the amplitude of oscillation at resonance is much lower and described by more of a flattened curve. The actual values of Q in real physical situations vary greatly. Since the classical understanding of atomic radiation essentially just involves the concept of linear oscillation, we may also define Q values for some atomic systems.