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)) H-atom in ground state (13.6eV) are excited by monochromatic radiation of photon of energy 12.1 eV. The number of spectral lines emitted in H-atom will be