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?

Conservation of Energy,  Photon →e++e−+energy EP=Ere++Ere−+K.E. of both (e−+e+)Rest mass energy of e+ or e−=mc2=(11823)931.5MeV=0.51MeV Hence, EP=0.51+0.51+0.19×2=1.40MeV