Spectrum of sodium : D lines : Easy learning :

The characteristic wavelengths of electromagnetic radiation that is emitted or absorbed by an object or substance, atom, or molecule. There are three types of spectrum .
1 . continuous spectrum .
2 . absorption line spectrum .
3 . emission line spectrum .
If the spectrum has no gaps its continuous spectrum as in rainbow it goes from red to violet without gap so its a continuous spectrum . when we look at the Sun's light and analyze it in detail, we find that there are gaps where we see nothing at all ! The gaps in the Sun's spectrum are called absorption lines .
The emission spectrum is the frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state.
Fraunhofer lines are a set of spectral lines observed as dark features in the optical spectrum of the sun . Fraunhofer labelled them from A to K .
The D lines are two close, prominent lines in the yellow region of the spectrum of neutral sodium at wavelengths (approximately 589 nm and 589.6 nm). The D lines are emitted by electron transitions from the 3p to the 3s levels of sodium.
Each element emission spectrum is distinct because each element has a different set of electron energy levels .

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