Use the Planck expression to find the energy of a photon.
[tex]E=hf[/tex]Where h = 6.63x10^-34 J*s and f = c/lambda.
[tex]\begin{gathered} E=6.63\times10^{-34}J\cdot s\cdot\frac{c}{\lambda}=6.63\times10^{-34}J\cdot s\cdot\frac{3\times10^8\cdot\frac{m}{s}}{724.4\times10^{-9}m} \\ E=2.75\times10^{-19}J \end{gathered}[/tex]Now, transform the energy from Jules to electron-volts. We know that 1 electron-volt equals 1.60x10^-9 J.
[tex]E=2.75\times10^{-19}J\cdot\frac{1eV}{1.6\times10^{-19}J}=1.72eV[/tex]Therefore, the energy of the photon with the given wavelength is 1.72 eV.
