An atom makes a transition from a state of energy E to one of lower energy E. Which of the following gives the wavelength of the radiation emitted in terms of the Planck's constants h and the speed of light c?

By quantum theory of radiation, the energy change E between energy levels is proportional to the frequency of electromagnetic radiation  and is given by the equation below△E=hf=hcλ where h is the Planck's constant, c is the speed of light and λ is the wavelength of the radiation.Now, energy change by transition from E3 to E1 is given by△E=E2-E1   ....(1)From (1) and (2), we haveE2-E1=hcλ⇒λ=hc(E2-E1)