Answer
657nm
Explanation
wavelength= 1/wavenumber
but wavenumber = RH×Z^2( (1/n1^2)-(1/n2^2))
where RH is the Rydberg constant = 1.0976×10^7
Z is the atomic number
n1 & n2 are orbitals
=(1.0976×10^7)×(1^2)×(1/4-1/9)
=1524444.444
but wavelength= 1/wavenumber
Therefore wavelength = 1/1524444.444
= 6.5597× 10^-7m
≈ 657nm
