1. Emission spectra are produced by thin gases in which the atoms do not experience many collisions. The emission lines correspond to photons of discrete energies that are emitted when excited atomic states make transitions back to lower-lying levels.

2. A continuum spectrum results when the gas pressures are higher, so that lines are broadened by collisions between the atoms. We may view a continuum spectrum as an emission spectrum in which the lines overlap with each other and can no longer be distinguished as individual emission lines.

3. An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the spectrum.