Relativistic equation of energy:E2=p2c2+m2c4…(1) Here, p2c2= kinetic energy of photonm02​c4= internal energy of photonWe know photons have zero rest mass. Therefore, m0​=0 Substituting the value of m0​=0 in equation (1), we get:E=pc Thus, the energy of a photon should be called its kinetic energy.