# Planck's Constant

### light energy physics radiation

Planck's constant relates the **energy** (E) of a **photon** with the **frequency** of **light**. Moreover, Planck's constant allows the 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, Planck's constant (*h* = 6.626 10^{-34 }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 Planck's 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 sixteen. By the age of twenty-one he had earned a doctorate in **physics**. While a graduate student, Planck studied **entropy** and the applications of the second law of **thermo-dynamics**. When Planck started his studies in physics, Newtonian or classical physics seemed fully explained. In fact, Planck's advisor claimed that there was essentially nothing new to discover in physics. Despite such warnings, Planck choose to study physics. Planck's 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 were 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 because—although atoms are constantly vibrating and generating electromagnetic waves—when 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 Planck's reservations, Albert Einstein's subsequent Nobel Prize winning work on the **photoelectric effect** was heavily based on Planck's theory and described light as being composed of photons, each with an energy equal to Planck's constant times the frequency of the light.

Light is now understood as having both photon (particle) and wave-like properties.

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

Planck's constant, combined with the speed of light, and the universal gravitational constant (G), can yield a quantity with the dimensions of **time** (5.38 x 10^{-44} 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 Planck's constant also improves the precision of related atomic constants, such as **proton mass**, **electron** mass, elementary charge, and **Avogadro's 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

Gribbin, John. *Q is for Quantum: An Encyclopedia of Particle* *Physics.* New York: The Free Press, 1998.

Griffiths, D.J. *Introduction to Quantum Mechanics.* Prentice-Hall, Inc. 1995.

Jackson, J.D. *Classical Electrodynamics.* John Wiley and Sons, 1998.

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.

K. Lee Lerner

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