Diffrentiate between absorption & emission spectra

absorption lines are where light has been absorbed by the atom thus you see a dip in the spectrum whereas.

emission spectra have spikes in the spectra due to atoms releasing photons at those wavelengths.

explanation:

To be sure that the color is due to sodium, do a flame test. This is used to identify the presence of small number of metal ions in a compound. For sodium, the brilliant color of bright yellow is a result of an excited electron in a very high level of energy and fall to a lower level called the ground state. 

In addition, metals have different jump sizes in the energy level and this means that each different metal will have a different pattern of spectral lines, which results to different flame color.