An atom emits a spectral line of wavelength λ when an electron makes a transition between levels of energy E1 and E2. Which expression correctly relates λ, E1 and E2?

By quantum theory of radiation, the energy change ∆E between energy levels is proportional to the frequency of electromagnetic radiation f and is given byΔE=hf=hcλ Hence, λ=hcΔE=hcE1−E2