People used to think that when light hits a cable next to another cable, electrons jumped from one to another. People thought it happened because waves of light made atoms vibrate until they ejected an electron. But Einstein thought it could be wave packets named photons. He explained this by shining light on a metal plate and knocking off electrons and hitting a nearby collector. Some of the energy was needed to free the electron, the energy was bound and the rest of the energy gone to the kinetic of the electron.