A particle of light is called a photon, which is a small particle made up of electromagnetic radiation. These particles have no charge or resting mass, but travel at the speed of light. The energy of photons cannot be divided, thus they are often referred to as energy packets.
Einstein predicted in 1905 that light might have an actual physical form, which was proved correct in the following decades. It was still several more decades before enough experiments were done for Einstein's ideas to become the standard with regard to light and photons. Einstein worked on his light theory for the rest of his life, but it took the creation of the field of quantum electrodynamics to answer all of the questions about light and his theories.