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In 1905, Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect. Einstein theorized that the energy in each quantum of light was equal to the frequency of light multiplied by a constant, later called the Planck constant. A ...
Einstein's explanation of the photoelectric effect extended the quantum theory which Max Planck had developed in his successful explanation of black-body radiation. Despite the greater fame achieved by his other works, such as that on special relativity, it was his work on the photoelectric effect that won him his Nobel Prize in 1921. [9]
The citation for Einstein's 1922 Nobel Prize very deliberately avoided all mention of light-quanta, instead stating that it was being awarded for "his services to theoretical physics and especially for his discovery of the law of the photoelectric effect".
To explain the photoelectric effect, Albert Einstein assumed heuristically in 1905 that an electromagnetic field consists of particles of energy of amount hν, where h is the Planck constant and ν is the wave frequency.
The source of Einstein's proposal that light was composed of particles (or could act as particles in some circumstances) was an experimental anomaly not explained by the wave theory: the photoelectric effect, in which light striking a metal surface ejected electrons from the surface, causing an electric current to flow across an applied voltage.
The photoelectric effect: Einstein explained this in 1905 (and later received a Nobel prize for it) using the concept of photons, particles of light with quantized energy. Robert Millikan's oil-drop experiment, which showed that electric charge occurs as quanta (whole units). (1909)
The notions of light as a particle resurfaced in the 20th century with the photoelectric effect. In 1905, Albert Einstein explained this effect by introducing the concept of light quanta or photons. Quantum particles are considered to have wave–particle duality.
As shown by Albert Einstein, [10] [53] some form of energy quantization must be assumed to account for the thermal equilibrium observed between matter and electromagnetic radiation; for this explanation of the photoelectric effect, Einstein received the 1921 Nobel Prize in physics. [54]