Blackbody radiation properties

Blackbody Radiation Engineering calculations of thermal radiation from surfaces are best keyed to the radiation characteristics of the blackbody, or ideal radiator. The characteristic properties of a blackbody are that it absorbs all the radiation incident on its surface and that the quality and intensity of the radiation it emits are completely determined by its temperature. The total radiative fliix throughout a hemisphere from a black surface of area A and absolute temperature T is given by the Stefan-Boltzmann law ... 

A hundred years ago it was generally supposed that all the properties of light could be explained in terms of its wave nature. A series of investigations carried out between 1900 and 1910 by Max Planck (1858-1947) (blackbody radiation) and Albert Einstein (1879-1955) (photoelectric effect) discredited that notion. Today we consider light to be generated as a stream of particles called photons, whose energy E is given by the equation... 

But light is also a particle. Some properties of light cannot be explained by the wave-like nature of light, such as the photoelectric effect and blackbody radiation (see Section 9.4), so we also need to think of light comprising particles, i.e. photons. Each photon has a direction as it travels. A photon moves in a straight line, just like a tennis ball would in the absence of gravity, until it interacts in some way (either it reflects or is absorbed). 

Blackbody radiation sources are accurate radiant energy standards of known flux and spectral distribulion. They are used for calibrating other infrared sources, detectors, and optical systems. The radiating properties of a blackbody source are described by Planck s law. Energy distribution... 

More than a century has passed since Planck discovered that it is possible to explain properties of the blackbody radiation by introducing discrete packets of energy, which we now call photons. The idea of discrete or quantized nature of energy had deep consequences and resulted in development of quantum mechanics. The quantum theory of optical fields is called quantum optics. The construction of lasers in the 1960s gave impulse to rapid development of nonlinear optics with a broad variety of nonlinear optical phenomena that have been... 

Blackbody radiation is achieved in an isothermal enclosure or cavity under thermodynamic equilibrium, as shown in Figure 7.4a. A uniform and isotropic radiation field is formed inside the enclosure. The total or spectral irradiation on any surface inside the enclosure is diffuse and identical to that of the blackbody emissive power. The spectral intensity is the same in all directions and is a function of X and T given by Planck s law. If there is an aperture with an area much smaller compared with that of the cavity (see Figure 7.4b), X the radiation field may be assumed unchanged and the outgoing radiation approximates that of blackbody emission. All radiation incident on the aperture is completely absorbed as a consequence of reflection within the enclosure. Blackbody cavities are used for measurements of radiant power and radiative properties, and for calibration of radiation thermometers (RTs) traceable to the International Temperature Scale of 1990 (ITS-90) [5]. 

Edgar Buckingham (1867-1940) was educated at Harvard and Leipzig, and worked at the U.S. National Bureau of Standards (now the National Institute of Standards and Technology or NIST) from 1905 to 1937. His fields of expertise included soil physics, gas properties, acoustics, fluid mechanics, and blackbody radiation. 

Any object will radiate energy in the form of electromagnetic radiation purely as a consequence of its temperature. The red glow of an electric heater and the bright white light of the tungsten filament in an incandescent light bulb are familiar examples. This radiation is referred to as blackbody radiation. The physical properties... 

The concept of blackbody is determining the basis for describing the radiation properties of real surfaces. The black body denotes an ideal radiative surface which absorb all incident radiation, being a diffuse emitter and emit a maximum amount of energy as thermal radiation for a given wavelength and temperature. The black body can be considered as a perfect absorber and emitter. 

Blackbody Idealized object that absorbs all electromagnetic radiation that is incident on it. The radiation properties of blackbody radiators are described by the Planck function. Planetary radio astronomers use the properties of blackbody radiators to describe the radiation from planets. 

An important property of a blackbody radiator is that its total radiant energy is a function only of its temperature that is, the temperature of a blackbody radiator uniquely determines the amount of energy that is radiated into any frequency band. Planetary radio astronomers make use of this property by expressing the amount of radio energy received from a planet in terms of the temperature of a blackbody of equivalent angular size. This concept is developed more fully in the following paragraphs. 

Thus far, we have discussed the ideal model in which the planets behave like blackbody radiators. This gives planetary astronomers a crude model from which they can estimate flux densities and search for departures. The planets would not be very interesting to study if they behaved like blackbodies since a single parameter, namely the temperature, could be used to define their radiation properties. More important, it is the departures from the simple model that allows radio astronomers to deduce the physical properties of the planets. 

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