A photon materializes into an electron-positron pair. The kinetic energy of the electron is found to be 0.19MeV. What was the energy of the photon?

$\begin{array}{l}\text{ Conservation of Energy, }\\ \text{ Photon }\to {\text{e}}^{+}+{\text{e}}^{-}+\text{energy }\\ {\text{E}}_{\text{P}}={\text{E}}_{{\text{re}}^{+}}+{\text{E}}_{{\text{re}}^{-}}+\text{K}.\text{E}.\text{ of both }({\text{e}}^{-}+{\text{e}}^{+})\\ {\text{Rest mass energy of e}}^{+}{\text{ or e}}^{-}=m{c}^2=\left(\frac{1}{1823}\right)931.5MeV=0.51\text{MeV}\\ {\text{ Hence, E}}_{\text{P}}=0.51+0.51+0.19\times 2=1.40\text{MeV}\end{array}$