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Dr. Albert Einstein: Theoretical Physics, Relativity and the Photoelectric Effect, 1935

It's (Photo) Electric!

Einstein's light quantum hypothesis helped to explain certain visible light behavior which could not be explained if visible light were understood to exist in the form of a wave, rather than in the form of tiny individual particles. One of these phenomena was known as the photoelectric effect. Scientists had observed that, when light hit metal, electrons were ejected from the surface of the metal. Einstein's light quanta could eject electrons from the surface of the metal by changing the energy states of the electrons they hit. Light quanta are little bundles of energy, and according to electron theory, electrons absorb energy. The act of absorbing energy takes an electron to a higher energy state, causing it to jump. When it returns to its state of rest, it emits the energy it has absorbed in the form of light. This results in the observable ejection of electrons from the metal's surface known as the photoelectric effect.