In the late 1800s, it was known that when a gas is excited using an electric discharge and the light emitted is viewed through a diffraction grating. This light can be studied with a spectrometer (A spectrometer is an instrument used to view and analyze a range of wavelengths of a given substance) and is found to comprise a series of lines.
This series of lines is called spectrum lines and the spectrum observed, consists not of a continuous band of light, but of individual lines with well-defined wavelengths. The wavelengths of these lines have been observed to be characteristic of the chemical element emitting the light. They are just as unique as a retina of the human eye which is unique to every individual.