Archimedes to Hawking: Laws of Science and the Great Minds behind Them

By Clifford A. Pickover | Go to book overview

PLANCK’S LAW OF RADIATION

Germany, 1900. The amount of energy at a particular wavelength radiated by a blackbody depends on the temperature of the body and the wavelength. Planck’s formulation is notable because it incorporates the earliest known practical application of quantum theory.

CROSS REFERENCE: ALBERT EINSTEIN, RUDOLF CLAUSIUS, KIRCHHOFF’S LAW OF THERMAL RADIATION, WIEN’S DISPLACEMENT LAW, THE STEFANBOLTZMANN RADIATION LAW, THE RAYLEIGH-JEANS LAW, AND WIEN’S RADIATION LAW.

In 1900, Sigmund Freud published The Interpretation of Dreams.
Hawaii officially became a U.S. territory and was granted self-
governance. American inventor Cornelius J. Brosnan filed for
a U.S. patent for a paperclip. He called his invention the
“Konaclip.”

Quantum theory, which suggests that matter and energy have the properties of both particles and waves, had its origin in pioneering research concerning hot objects that emit radiation. For example, imagine the coil on an electric heater that glows brown and then red as it gets hotter. More particularly, Planck’s Law of Radiation quantifies the amount of energy emitted (at a particular wavelength) by a particular kind of hot glowing object called a blackbody. A blackbody, discussed in “Kirchhoff’s Electrical Circuit and Thermal Radiation Laws” (see part III), is an object that emits and absorbs the maximum possible amount of radiation at any given wavelength and at any given temperature. I describe blackbodies further at the end of this section.

Thermal radiation is radiant energy emitted by an object as a result of the temperature of the object. The spectrum of the thermal radiation from a hot body is continuous over a range of wavelengths. Usually, the amount of radiation at any given frequency is different than at other frequencies. Many of the objects that we encounter in our daily lives emit a large portion of their radiation spectrum in the infrared, or far-infrared, portion of the spectrum, and this radiation is not visible to our eyes. However, as the temperature of a body increases, the dominant portion of its spectrum shifts so that we can see a glow from the object.

Even though such thermal radiation spans a range of wavelengths, scientists are often interested in the radiation emitted per unit wavelength, called monochromatic radiation, and the amount of such radiation depends on the temperature. For example, the Sun has an effective

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