Quantum Mechanics is humanity's attempt to explain the microscopic world of atoms and particles. More specifically it helps to explain actions of photons, what makes up light, and the reasons scientists were getting weird results when testing varying things from light. Finally Einstein (Right) suggested the Theory of wave-particle duality of light, which states that light will act as a particle at times and like a wave at other times. This gives it properties of energy, momentum (particle properties), but was massless.
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