Hydrogen gas can be induced to emit light when it is sealed in a glass tube and a high voltage (about 5,000 volts) is applied between electrodes at opposite ends of the tube.
When the light from this tube is passed through a prism or grating, a spectrum like that below is produced. This is a line source, which contains light at only a few narrow bands of wavelength.
The exact wavelengths of the lines can be determined from a graph of light intensity vs. wavelength like the one below. This spectrum arises from the internal structure of the hydrogen atoms.