Energy of an electron in the nth orbit of H-atom, $E_n$ = - $\frac{13.6}{n^2}$ eV Energy in the ground (n = 1) level, $E_1$ = - $\frac{13.6}{1^2}$ = - 13.6 eV Energy in the fourth (n = 4) level, $E_4$ = - $\frac{13.6}{4^2}$ = - 0.85 eV Energy radiated during emission ΔE = $E_4 - E_1$ = –0.85 – (–13.6) = 12.75 eV As hυ = ΔE ∴ Frequency, υ = $\frac{\Delta E}{h}$ = $\frac{12.75 \times 1.6 \times 10^{-19}}{6.6 \times 10^{-34}}$ = 3.078 × $10^{15}$ Hz Wavelength, λ = $\frac{c}{\upsilon}$ = $\frac{3 \times 10^8}{3.078 \times 10^{15}}$ = 0.9744 × $10^{-7}$ m = 974.4 Å