Chapter 7 - Quantum Theory and the Electronic Structure of Atoms - Questions & Problems - Page 316: 7.30

$3.027\times10^{-19}\,J$

$\lambda=\frac{c}{\nu}=\frac{ch}{\Delta E}$ $\implies \Delta E=\frac{ch}{\lambda}$ where $c$ is the speed of light, $h$ is the Planck's constant and $\lambda$ is the wavelength. Then, $\Delta E=\frac{(2.998\times10^{8}\,m/s)(6.626\times10^{-34}\,J\cdot s)}{656.3\times10^{-9}\,m}=3.027\times10^{-19}\,J$