A spectrum is formed when electrons move to different energy levels within an atom.
Photons (light-waves) can be absorbed by an atom when an electron moves from a lower energy level to a higher energy level, see figure F-2-4.
Only a photon of a specific energy (or wavelength) will be absorbed. Otherwise, the photon will just move through the atom. Energy carried by the absorbed photon is equal to the difference in the energy between the two energy levels the electron moved from and to. In this case:
Photons (light-waves) are emitted from an atom when an electron moves from a higher energy level to a lower energy level.
The energy (or wavelength) of the emitted photon depends on the energy levels of the atom!
In this example:
Because every element has a different set of atomic orbital energies, the spectrum of each element is unique. Emission and absorption lines of a chemical element are as unique as fingerprints! Below are some emission spectra of different chemical elements.
Every chemical element has a unique set of energy levels and depending on whether we see the element in absorption (as a cool gas in front of a hot dense source) or in emission (as excited gas) the spectral lines are always in the same exact place for that element. Figure F-2-7 shows sodium (Na) in (a) emission and (b) in absorption.