particle
In 1905, the physicist Albert Einstein developed a new theory about electromagnetic radiation. The theory is often called the wave-particle theory . It explains how electromagnetic radiation can behave as both a wave and a particle. Einstein argued that when an electron returns to a lower energy level and gives off electromagnetic energy, the energy is released as a discrete packet of energy. We now call such a packet of energy a photon . According to Einstein, a photon resembles a particle but moves like a wave. You can see this in the Figure below . The theory posits that waves of photons traveling through space or matter make up electromagnetic radiation.