Origin of Atomic Spectrum:

You would recall from your previously knowledge in which an atom holds a set of quantised energy stages which can be occupied through the electrons depending on the energy. The energy level along with the minimum energy is known as ground state although the ones of higher energy are referred to as the excited states.  In addition, electron within an energy level could undergo a transition to another stage of a higher energy through absorbing a photon of appropriate energy equivalent to the difference within the energies of the two levels. Instead the transition of an electron from a higher energy level to a level of lower energy is accompanied through the emission of the difference in energy within the form of a radiation of appropriate wavelength.  The spread of the wavelengths of the photons absorbed or emitted versus their intensities is known as a spectrum. The atomic spectroscopic techniques are based on the transitions amongst the quantised electronic energy levels caused through the absorption of radiation through the atoms in ground state or through the emission of radiation through the excited atoms within vapour phase.