Hello student,

Since a sodium has 11 electrons, and they are in an orbit so one of the electron is most likely to accept and emit energy.

To answer your query, we will use the principle of photoelectric effect. There is a U shaped glass tube with small amounts of neon, argon gases and sodium gas. Now when we will pass the current, the temperature of the gas increases and it vaporizes. So, there will be an electric discharge when electricity passes through the sodium vapors at high or low pressure. Due to all this, valence electron of the sodium atoms to excite to higher energy levels. These excited electrons emit light of wavelength of 590 nano meters.

This wavelength depicts the yellow light.

Thanks.