How is an emission spectrum of an element generated?
The frequencies of light that an atom can emit are
dependent on states the electrons can be in. When excited, an electron
moves to a higher energy level or orbital. When the electron falls back
to its ground level the light is emitted.
Answer is: When an electron in an atom returns from an excited state to a lower energy state.
Emission spectrum of a chemical element is the spectrum of frequencies emitted due to an atom making a transition from a high energy state to a lower energy state.
Emission spectrum has characteristic energy proportional to the frequency of the light.
Each element's emission spectrum is unique.