Hydrogen atom has a single electron. Its spectrum consists of series of spectral lines. Lyman series : Lyman Series consists of spectral lines corresponding to the transition of an electron from higher energy orbits n = 1 and n₂ = 2,3,4… These lines belong to Ultraviolet region. 

Balmer series : Balmer series consists of spectral lines emitted during transitions of electrons from higher energy orbits to the second orbit. n₁ = 2 and n₂ = 3,4,5… .These lines lie is the visible region. 

Paschen series : Paschen series consists of spectral lines emitted when electron jumps higher energy orbits to the third orbit n₁ = 3 and n₂ = 4,5,6 … .These lines lie in the infrared region. 

Brackett series : Brackett series consists of spectral lines emitted during transitions of electrons from higher energy orbits to fourth orbit. n₁ = 4 and n₂ = 5,6,7… 

Pfund series : Pfund series consists spectral lines emitted during transition of electrons from higher energy orbits to the fifth orbit. . n₁ = 5 and n₂ = 6,7,8… . 

These lines lie in infrared region . The transition from (n₁ + 1) to n₁ corresponding to I^st member or longest wavelength of the series. The transition from (infinity) state to ‘n₁’ state corresponds to the last number or series limit or shortest wavelength of the series.