Planck's Constant, fundamental physical constant, symbol h. It was first discovered (1900) by the German physicist Max Planck. Until that year, light in all forms had been thought to consist of waves. Planck noticed certain deviations from the wave theory of light on the part of radiations emitted by so-called blackbodies, or perfect absorbers and emitters of radiation. He came to the conclusion that these radiations were emitted in discrete units of energy, called quanta. This conclusion was the first enunciation of the quantum theory. According to Planck, the energy of a quantum of light is equal to the frequency of the light multiplied by a constant. His original theory has since had abundant experimental verification, and the growth of the quantum theory has brought about a fundamental change in the physicist's concept of light and matter, both of which are now thought to combine the properties of waves and particles. Thus, Planck's constant has become as important to the investigation of particles of matter as to quanta of light, now called photons. The first successful measurement (1916) of Planck's constant was made by the American physicist Robert Millikan. The present accepted value of the constant is h = 6.626 × 10^-34 joule-second in the meter-kilogram-second system.

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