The Origins of spectrum are:
i. When sun rays passes through the prism it splits into different colours rays.
ii. In an atom, when electron is transferred from higher energy state to lower energy state it emits electromagnetic radiation. When this radiation is analyzed in spectrometer, collections of bright lines are seen. It is called emission or light spectrum. Conversely, when electron is transferred from lower energy state to higher state absorption or dark spectra is seen.
iii. In lower pressure, if electrical discharge or electromagnetic ray is applied in gas, electrons of the atom is become excited. When returns to its normal state it radiate rays. This spectrum is called atomic emission spectrum.