In 1905, Albert Einstein made two separate, and very significant, contributions to our understanding of the nature of matter. If that is true, then the interactions of light and matter are interactions of single photons with single electrons in single atoms. By 1916, experimental studies had accurately confirmed his prediction. The photolectric effect is shown in the Figure below.
Energy is conserved for any body of any mass moving at any velocity.
The kinetic energy of a body will be a significant part of the mass-energy total of the body only if the body is moving at an appreciable fraction of the speed of light. Only electrons and nuclear particles have been found or made to travel at such speeds. Let us calculate the mass of an electron moving at one-half the speed of light. The rest mass of an electron is 0.910954 x 10 -20 kg. The properties of matter and those of electromagnetic radiation are both properties of the same real phenomenon.