Why do exited hydrogen atoms emit a light spectrum that consists of a few lines rather than a continuous spectrum?

Answer 1

All elements do this. The emitted light is a result of electrons that have been excited (raised to a higher energy level) falling back to their minimimum energy condition (the ground state) or to any lower available energy state. All of the energy states in an atom are quantized - that is, there are only certain acceptable energy levels at which the electrons can exist. and the energy levels of these states are unique for each element. a simple way of thinking about tis is that the different energy levels are dependent on how many protons and neutrons are in the nucleus. Thus each element (in fact each different isotope) will emit a characteristic 'spectrum' or group of narrowly defined energy emissions as the excited electrons fall back to lower energy levels. these are called spectrial 'lines' because they actually appear as narrow bright lines when the light from the excited element is diffracted by a prism, as opposed to a continuous spectrum (not individual well defined lines) that you would see if you put the light from say, a white hot piece of metal, through a prism. The typical way to excite a gas like hydrogen is put it in a tube and send electricity through it. The electric discharge constantly is bumping electrons up to higher energy levels - in fact sometimes ripping them off the atom completely, so electrons are also constantly falling back to the ground state or lower eneergy states. Note also that pure gasses also only ABSORB energy on these discrete levels, so if you shine a continuous spectrum light like from an incandescent light bulb through a pure gas and then diffract the result throgh a prism, you will see dark bands corresponding to the light bands that you would see if you excited the same gas. From this the early physicists inferred a lot about the structure of the miniscule atoms which cannot be seen in any way by the human eye - only their interactions like this can be detected.

Answer 2

When exited hydrogen atoms release energy and transition to lower energy levels, they emit photons with specific energies corresponding to the differences in energy levels. This results in the emission of discrete wavelengths of light, creating a line spectrum with specific spectral lines, rather than a continuous spectrum. Each line corresponds to a specific electronic transition within the hydrogen atom.