Find out the longest wavelength of absorption line for hydrogen gas containing atoms in ground state.

For longest wavelength ΔE should be smallest i.e. transition occurs from n = 1 to n = 2

i.e. 1/λ = 109673 cm-1 x 12 [1/12 – 1/22]

1/λ = 109673 x 3/4 cm-1

λ = 1.2157 x 10-5 cm = 121.6 nm