Chapter 7 - Electronic Structure - Questions and Exercises - Exercises - Page 285: 7.34

$3.1\times10^{-19}J$

E=hν = hc/λ h = $6.626\times10^{-34}Js$ c = $2.997925\times10^{8}ms^{-1}$ λ = 640 nm = $640\times10^{-9}m$ E = $\frac{6.626\times10^{-34}Js\times2.997925\times10^{8}ms^{-1}}{640\times10^{-9}m}$ = $3.1\times10^{-19}J$