The frequency of radiations emitted when electron falls from n = 4 to n = 1 in H-atom would be (Given E1 for  H=2.18  × 10−18 J  atom−1 and  h=6.625  × 10−34 Js.)

Ionisation energy of H=2.0  × 10−18 J  atom−1∴  E1 energy  of  1st  orbit  of H−atom =2.18  × 10−18 J  atom−1∴      En = −2.18  × 10−18n2Z=1  for   H−atomΔE=En−E1=  −2.18  × 10−1842−−2.18  × 10−1812ΔE=  −2.18  × 10−18  ×   −1516=+2.0437  × 10−18  ×   J   atom−1v=ΔEh=2.04 × 10−18 6.626 × 10−34=3.084 × 1015  s−1  atom−1.