BLACKBODY CONCEPT:
A blackbody is an imaginary body that, when cool, absorbs all the radiant energy falling on its surface so that it is black in color; when hot, the blackbody emits energy with 100 percent efficiency. (Real matter is generally less than 100 percent efficient when it radiates.)
For our purposes the most important feature of the blackbody is the way in which emitted radiant energy is spread in wavelength, or the spectral energy distribution. Scientists have found that the distribution of energy depends only on the blackbody'S temperature and not on its chemical composition. Note how the amount of radiant energy emitted by a blackbody varies with wavelength in a very recognizable way, even for different temperatures. The emission of radiant energy (or the brightness at each wavelength) covers a continuous range of wavelengths so that the spectrum of a blackbody is a continuous spectrum; that is, there are no color bands missing from its spectrum. At room temperature, lampblack (a finely powdered black soot) is very close to being a blackbody because it absorbs almost all the radiation incident upon it and reflects very little. Fortunately, the radiation emitted by stars tends to be much like that emitted by a blackbody.
In 1900 the German physicist Max Planck (1858-1947) derived a mathematical expression, now called Planck's law, that describes the distribution of brightness in the spectrum of a blackbody. There are two other distinguishing characteristics of the spectrum of blackbody radiation: First, the energy emitted by the blackbody is greater at every wavelength as the temperature increases. Thus the total amount of radiant energy emitted increases with increasing temperature, which is known as the Stefan-Boltzmann law. Second, the greatest amount of radiation is fou nd toward shorter wavelengths (blue end of the spectrum) as the temperature increases. This is known as Wien's displacement law.
The significance of the blackbody-radiation lawsPlanck's law, the Stefan-Boltzmann law, and Wien's law-is that bodies that emit electromagnetic radiation because they are hot, such as stars, do so much like a blackbody. Thus the blackbody-radiation laws are powerful diagnostic tools for measuring the temperature of these thermal sources of radiation. For the study of bodies that emit radiation not because they are hot (called nonthermal sources of radiation) but because of some selective physical processes, the blackbody-radiation laws are of no use. Fortunately, most of the celestial bodies-all the stars-are thermal sources of radiation and emit much like a blackbody. Some everyday examples of thermal sources of radiation are an incandescent light bulb, the burner on an electric stove, and the flame of a cutting torch. Examples of nonthermal sources are a fluorescent light, lightning, and a television screen.
A blackbody is an imaginary body that, when cool, absorbs all the radiant energy falling on its surface so that it is black in color; when hot, the blackbody emits energy with 100 percent efficiency. (Real matter is generally less than 100 percent efficient when it radiates.)
For our purposes the most important feature of the blackbody is the way in which emitted radiant energy is spread in wavelength, or the spectral energy distribution. Scientists have found that the distribution of energy depends only on the blackbody'S temperature and not on its chemical composition. Note how the amount of radiant energy emitted by a blackbody varies with wavelength in a very recognizable way, even for different temperatures. The emission of radiant energy (or the brightness at each wavelength) covers a continuous range of wavelengths so that the spectrum of a blackbody is a continuous spectrum; that is, there are no color bands missing from its spectrum. At room temperature, lampblack (a finely powdered black soot) is very close to being a blackbody because it absorbs almost all the radiation incident upon it and reflects very little. Fortunately, the radiation emitted by stars tends to be much like that emitted by a blackbody.
In 1900 the German physicist Max Planck (1858-1947) derived a mathematical expression, now called Planck's law, that describes the distribution of brightness in the spectrum of a blackbody. There are two other distinguishing characteristics of the spectrum of blackbody radiation: First, the energy emitted by the blackbody is greater at every wavelength as the temperature increases. Thus the total amount of radiant energy emitted increases with increasing temperature, which is known as the Stefan-Boltzmann law. Second, the greatest amount of radiation is fou nd toward shorter wavelengths (blue end of the spectrum) as the temperature increases. This is known as Wien's displacement law.
The significance of the blackbody-radiation lawsPlanck's law, the Stefan-Boltzmann law, and Wien's law-is that bodies that emit electromagnetic radiation because they are hot, such as stars, do so much like a blackbody. Thus the blackbody-radiation laws are powerful diagnostic tools for measuring the temperature of these thermal sources of radiation. For the study of bodies that emit radiation not because they are hot (called nonthermal sources of radiation) but because of some selective physical processes, the blackbody-radiation laws are of no use. Fortunately, most of the celestial bodies-all the stars-are thermal sources of radiation and emit much like a blackbody. Some everyday examples of thermal sources of radiation are an incandescent light bulb, the burner on an electric stove, and the flame of a cutting torch. Examples of nonthermal sources are a fluorescent light, lightning, and a television screen.
