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

A Quantum Leap

In 1905, Einstein challenged the concept that visible light, one form of the electromagnetic wave, always behaved as a continuous wave. Einstein argued that in certain cases light behaves as individual particles. He called these particles "light quanta," and said that each "light quanta" carries a "quantum," meaning a fixed quantity of energy. A light beam is thus composed of many "light quanta" which are observed as one continuous wave. The total energy of a light beam, Einstein said, is the sum total of the individual energies of the distinct "light quanta." Today, these "light quanta" are called "photons." Theories that treat total energy as "quantized" (meaning that total energy is calculated by adding together the fixed energies of the individual "quanta" of which the overall energy is composed) are known as quantum theories.