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When a high potential is applied to hydrogen gas at low pressure in a discharge tube, it starts emitting a bright light. It is separated into several radiations and forms a spectrum upon passing through a prism or grating. The spectrum consists of separate lines corresponding to different wavelengths. This is called Hydrogen atomic spectrum. 

The spectral lines are formed due to electronic transitions from one energy level to another. These lines are divided into five series according to the range of wavelengths as follows. 

Spectral series       Spectral region         n1         n2 

1. Lyman series Ultra-violet          1      2,3,4,5,6,7,_ _ _ _ 

2. Balmer series Visible                  2      3,4,5,6,7,_ _ _ _ 

3. Paschen series near infra-red          3       4,5,6,7,_ _ _ _ 

4. Brackett series infra-red                  4       5,6,7,_ _ _ _ 

5. Pfund series       far infra-red          5       6,7,_ _ _