In a sample of H - atom, electrons make transition from 5 th  excited state to ground state, producing all possible types of photons, then number of lines in infrared region are:

Step-by-step solution

Infrared lines = Total lines - visible lines - UV lines = 6 ( 6 - 1 ) 2 - 4 - 5    = 15 - 9 = 6 (visible lines = 4    6 → 2 , 5 → 2 , 4 → 2 , 3 → 2 ) (UV lines = 5    6 → 1 , 5 → 1 , 4 → 1 , 3 → 1 , 2 → 1 )