The ionisation energy of hydrogen atom is 13.6 eV. Following Bohr's theory, the energy corresponding to a transition between the 3rd and the 4th orbit is

3.40 eV 1.51 eV 0.85 eV 0.66 eV

0.66 eV

$E_3=-\frac{13.6}{9}=-1.51 ~eV ; \quad E_4=-\frac{13.6}{16}=-0.85 ~eV$ ∴  $E_4-E_3=0.66 ~eV$