A photon of energy 10.2 eV corresponds to light of wavelength λ0. Due to an electron transition from n = 2 to n = 1 in a hydrogen atom, light of wavelength λ is emitted. If we take into account the recoil of the atom when the photon is emitted.
1
λ = λ0
2
λ < λ0
3
λ > λ0
4
the data is not sufficient to reach a conclusion