Excited atoms emits radiations consisting of only certain discrete frequencies or wavelengths. In spectroscopy it is often more convenient to use frequencies or wave numbers than are proportional to energy and spectroscopy involves transitions between energy levels. The line spectrum shown by and mono electronic excited atom (a finger pring of an atom) can be given as 1/(lamda)=barv=R_(A)Z^(2)[1/(n_(1)^(2))-1/(n_(2)^(2))] When Z is atomicnumber of mono electronic atom and n_(1),n_(2) are integers and if n_(2)gtn_(1), then emission spectrum is noticed and if n_(2)ltn_(1), then absorptioni spectrum is noticed. Every line in spectrum can be represented as differences of two terms (R_(A)Z^(2))/(n_(1)^(2)) and (R_(A)Z^(2))/(n_(2)^(2)) The given diagrma indicates the energ levels of certain atom. Where an electron moves from 2E. level to E level, a photon of wavelength of photon emitted during its transition from (4E)/3 level to E level.