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PlanckS constant

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Plancks constant relates the energy (E ) of a photon with its frequency. It allows precise calculation of the energy of light emitted or absorbed and thereby permits the determination of the actual energy of the photon. Along with constant for the speed of light, Plancks constant (h = 6.626 × 1034 joule-second in the meter-kilogram-second system of measurements) is a fundamental constant of nature.

At the beginning of the twentieth century, German physicist, Maxwell Planck proposed that atoms absorb or emit electromagnetic radiation only in certain units or bundles of energy termed quanta. The concept that energy existed only in discrete and defined units seemed counter-intuitive, that is, outside the human experience with nature. Accepting his experimental results regarding the radiation emitted by an object as its temperature increases, Planck developed a quantum theory that accounts for a wide range of physical phenomena.

Prior to Plancks work, electromagnetic radiation (light) was thought travel in waves with an infinite number of available frequencies and wavelengths. Planck determined that energy of light was proportional to its frequency. As the frequency of light increases, so does the energy of the light.

Planck began his university studies at the age of 16. By the age of 21 he had earned a doctorate in physics. While a graduate student, Planck studied entropy and the applications of the second law of thermodynamics. When Planck started his studies in physics, Newtonian or classical physics seemed fully explained. In fact, Plancks advisor claimed that there was essentially nothing new to discover in physics. Despite such warnings, Planck chose to study physics. Plancks talents and dedication were recognized and upon the death of his mentor Gustav Robert Kirchoff, Planck became a professor of theoretical physics at the University of Berlin where he did the major portion of his work regarding the relationship of light energy to light wavelength. Planck was able to measure radiation from heated bodies becausealthough atoms are constantly vibrating and generating electromagnetic waveswhen heated, an atom vibrates at higher frequencies and gives off radiation at higher levels of energy.

Planck admitted that he did not fully understand quantum theory. In fact he regarded it as only a mathematical aberration or temporary answer until a more intuitive or common sense answer was found. Despite Plancks reservations, Albert Einsteins subsequent Nobel Prize winning work on the photoelectric effect was heavily based on Plancks theory and described light as being composed of photons, each with an energy equal to Plancks constant times the frequency of the light.

Light is now understood as having both photon (particle) and wavelike properties.

In 1916, American physicist Robert Millikans experiments gave the first precise calculation of Plancks constant. Modern laboratories, including the national Institute of Standards and Technology strive for more precise values for Plancks constant because it is so fundamental to applications of modern physics and chemistry.

Plancks constant, combined with the speed of light, and the universal gravitational constant (G ), can yield a quantity with the dimensions of time (5.38 × 1044 seconds). This quantity is called Planck time a very important concept in cosmology (the study of the origin of the cosmos). Because it is a fundamental constant, more precise values for Plancks constant also improves the precision of related atomic constants, such as proton mass, electron mass, elementary charge, and Avogadros number.

See also Atomic models; Blackbody radiation; Cosmic ray; Electromagnetic spectrum; Electromagnetism; Quantum mechanics; Spectral classification of stars; Spectral lines; Spectroscopy; Spectrum; Virtual particles.

Resources

BOOKS

Lide, D.R., ed. CRC Handbook of Chemistry and Physics. Boca Raton: CRC Press, 2001.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.

OTHER

University of Colorado. Plancks Constant and the Energy of a Photon. <http://www.colorado.edu/physics/2000/quantumzone/photoelectric2.html> (accessed November 15, 2006).

K. Lee Lerner

Planck’S constant

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