Excited atoms emit radiations consisting of only certain discrete frequencies or wavelengths. In spectroscopy it is often more convenient to use frequencies or wave numbers than wavelength because frequencies and wave numbers are proportional to energy and spectroscopy involves transitions between different energy levels. The line spectrum shown by a mono electronic excited atom (a finger print of an atom) is called atomic spectrum.
$\frac{1}{\mathrm{\lambda }}={\mathrm{Z}}^2\mathrm{R}\left[\frac{1}{{\mathrm{n}}_{\mathrm{I}}^2}-\frac{1}{{\mathrm{n}}_2^2}\right]$