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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | |
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First Edition Preface Second Edition Preface Acknowledgments
Section A A index 51-59 of 917 terms
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absorptance—One minus the sum of reflectance and transmittance. Compare absorbance.
absorption band—A connected series of closely spaced or overlapping absorption lines. Absorption bands are common features in the absorption spectra of polyatomic gases. Absorption bands arise when absorbed photons are converted to simultaneous changes in more than one of the electronic, vibrational, or rotational molecular energy states. Changes to vibrational energy are usually accompanied by changes in rotational energy and give rise to vibrational-rotational absorption bands. Similarly, changes to molecular electron levels yield electronic-vibrational-rotational absorption bands.
absorption coefficient—A measure of the extinction due to absorption of monochromatic radiation as it traverses a medium. Usually expressed as a volume absorption coefficient, kv, with units of reciprocal length (i.e., area per unit volume), but also as a mass absorption coefficient, km, with units of area per unit mass. The two are related by kv = km, where is the density of the absorber. See also absorption optical thickness.
absorption cross section—The area that, when multiplied by the irradiance of electromagnetic waves incident on an object, gives the radiant flux absorbed and dissipated by the object. Customary usage in radar describes the absorption cross section of an object as the area that, when multiplied by the power density of incident plane-wave radiation, gives the power absorbed and dissipated by the object. The extinction cross section of an object is the sum of the absorption cross section and the scattering cross section. For a medium consisting of a dispersion of absorbing objects through which radiation propagates, the volume absorption coefficient (units: m2m−3 or m−1) at a given location in the medium is the sum of the absorption cross sections of all the objects in a unit volume centered at that location.
absorption hygrometer—(Sometimes called chemical hygrometer.) A type of hygrometer that measures the water vapor content of the atmosphere by means of the absorption of vapor by a hygroscopic chemical. The amount of vapor absorbed may be determined in an absolute manner by weighing the hygroscopic material (see gravimetric hygrometry), or in a nonabsolute manner by measuring a physical property of the substance that varies with the amount of water vapor absorbed. The humidity strip, the carbon-film hygrometer element, and the thin-film capacitor are examples of the latter.
absorption line—A line of finite width in the absorption spectrum. Absorption lines are characterized by their central wavelength, line intensity, and line width.
absorption loss—The quantity of water that is lost during the initial filling of a reservoir because of absorption by soil and rocks.
absorption optical thickness—The component of the total optical thickness that is due to absorption only.
absorption spectrum—The detailed dependence on wavelength of the intensity of radiation absorbed by a given medium. Absorption spectra of gases are typically composed of discrete spectral lines and bands of overlapping lines that depend on the molecular or atomic composition of the absorbing substance and that may be used to identify it uniquely. When measuring the absorption spectrum, the medium should be considerably colder than the source of incident radiation (which should also be continuous in wavelength), in order to clearly distinguish the absorption spectrum from the analogous emission spectrum.
absorption—1. The process in which incident radiant energy is retained by a substance. A further process always results from absorption, that is, the irreversible conversion of the absorbed radiation into some other form of energy within and according to the nature of the absorbing medium. The absorbing medium itself may emit radiation, but only after an energy conversion has occurred. See attenuation. 2. The taking up or assimilation of one substance by another. The substances may combine chemically, or the absorption may just correspond to a physical solubility. See Henry's law.
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