Question

What is the difference between emssion and absorption spectra of sodium?

Answer 1

Emission spectra: When the light emitted directly from a source is examined with a spectrometer, the emission spectrum is obtained. Every source has its own characteristics emission spectrum.

Or, when the excited electrons fall back from higher energy level to their stationary levels,energy is emitted which results in emission spectra. The emitted wavelength will exactly correspond to the energy difference between the two levels. When the light coming from the atom is analysed, it will consist of bright lines of particular wavelengths only on a dark background because other wavelengths are absent.

The sodium spectrum is dominated by the bright doublet known as the Sodium D-lines at $589.0 \ nm$ and $589.6\ nm$ . From the energy level diagram it can be seen that these lines are emitted in a transition from the $3p$ to the $3s$ levels.

Absorption Spectra : When the light emitted from a source is made to pass through an absorbing material and then examined with a spectrometer, the obtained spectrum is called absorption spectrum. It is characteristic of the absorbing substance.

When an electron having energy $E_1$ jumps to a higher level associated with $E_2$ , it must absorb the difference in energy $(E_2-E_1)$ . This corresponds to a particular wavelength. When an electron in level $E_2$ jumps to a higher level $E_3$ , it must absorb the difference in energy $(E_3-E_2)$ . This corresponds to another wavelength. Thus when atoms are illuminated using white light, certain particular wavelengths are absorbed (or subtracted) from the continuous spectrum. We end up with a spectrum missing certain lines (corresponding to wavelength that was absorbed). Black lines on a bright background. This is an absorption spectrum.