Explain emission lines and absorption lines.

Explanation

When an electron in an atom transits from a higher energy state to a lower energy state then in both the states they produce light (photon) with corresponding energy difference which produces coloured bright lines. These coloured lines get a spectrum of wavelengths or frequency that are sorted according to the pattern. The lines of the spectrum obtained in this way are called emission lines.
When an electron in a atom transits from a lower energy state to a higher energy state so the electron absorbs the photon having the same energy difference between two states. This process is called absorption.
Thus if photons with a continuous range of frequencies pass through a rarefied gas and then are analysed with a spectrometer, a series of dark spectral absorption line appear in the continuous spectrum.
The dark lines indicate the frequencies that have been absorbed by the atoms of the gas.