Why there are so many lines …

A hydrogen atom has only one electron it contains a large number of shells so when this single electron jumps from one shell to another,a photon is emitted,and the energy difference of the shells causes different wavelengths to be released. Hence mono-electric hydrogen has many spectrum lines.Transitions of electron in the hydrogen atom are 1.LYMAN series.2.Balmer.3.Paschen.4.Bracket.5.Pfund.6.Humphire series.

Hydrogen atom has one electron present in the first shell.  When energy is given to the atom the electron gets excited and moves to a higher orbit.  Excited states are unstable and quickly decay to the ground state i.e. the electron moves back to the first shell.  This movement can either be in one step or a series of steps. The electron emits energy of a certain frequency when it falls from one energy level to another and for each movement it emits electromagnetic radiation of a certain frequency giving rise to a series of spectral lines each corresponding to the fall of excited electrons from one orbit to another.  The spectral lines are divided into series depending on the shell the excited electron falls back to e.g. electrons that fall to the first shell from any of the higher shell, the spectral lines are called Lyman series. Balmer is the name for all lines where electrons move to shell 2 from higher orbits.