why we get only two yellow lines in case of sodium spectrum?

 Line Emission Spectrum

• When atoms of any substance excited by supplying some amount of energy, they emit radiations which form bright lines in spectrum.
• These lines are not continuous but separated by dark line. Such type of spectrum is termed as line spectrum.
• Lines in line spectrum can be of different colors which depends on substance and temperature.
• Generally the light from neon sign lamps, sodium vapor lamp and mercury vapor lamp gives line spectrum.
• For example, sodium shows two yellow lines at the wave length 589.0nm and 589.6nm which are clearly visible. Hence when an atom drops from higher energy level to lower energy level, it emits some amount of energy in the form of radiation of exact energy.
• For example, when a hydrogen atom drops from the second energy level to the first energy level, it gives off a radiation whose energy is equal to the difference of energy of both energy levels. The bright line at the specific wavelength corresponds to the energy difference resulting in a specific color giving off an emission spectrum.