absorption spectrum

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spectrum, arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph). Physicists often find it useful to separate a beam of particles into a spectrum according to their energy.

Continuous and Line Spectra

Dispersion, the separation of visible light into a spectrum, may be accomplished by means of a prism or a diffraction grating. Each different wavelength or frequency of visible light corresponds to a different color, so that the spectrum appears as a band of colors ranging from violet at the short-wavelength (high-frequency) end of the spectrum through indigo, blue, green, yellow, and orange, to red at the long-wavelength (low-frequency) end of the spectrum. In addition to visible light, other types of electromagnetic radiation may be spread into a spectrum according to frequency or wavelength.

The spectrum formed from white light contains all colors, or frequencies, and is known as a continuous spectrum. Continuous spectra are produced by all incandescent solids and liquids and by gases under high pressure. A gas under low pressure does not produce a continuous spectrum but instead produces a line spectrum, i.e., one composed of individual lines at specific frequencies characteristic of the gas, rather than a continuous band of all frequencies. If the gas is made incandescent by heat or an electric discharge, the resulting spectrum is a bright-line, or emission, spectrum, consisting of a series of bright lines against a dark background. A dark-line, or absorption, spectrum is the reverse of a bright-line spectrum; it is produced when white light containing all frequencies passes through a gas not hot enough to be incandescent. It consists of a series of dark lines superimposed on a continuous spectrum, each line corresponding to a frequency where a bright line would appear if the gas were incandescent. The Fraunhofer lines appearing in the spectrum of the sun are an example of a dark-line spectrum; they are caused by the absorption of certain frequencies of light by the cooler, outer layers of the solar atmosphere. Line spectra of either type are useful in chemical analysis, since they reveal the presence of particular elements. The instrument used for studying line spectra is the spectroscope.

The Quantum Explanation of Spectral Lines

The explanation for exact spectral lines for each substance was provided by the quantum theory. In his 1913 model of the hydrogen atom Niels Bohr showed that the observed series of lines could be explained by assuming that electrons are restricted to atomic orbits in which their orbital angular momentum is an integral multiple of the quantity h/2π, where h is Planck's constant. The integer multiple (e.g., 1, 2, 3 …) of h/2π is usually called the quantum number and represented by the symbol n.

When an electron changes from an orbit of higher energy (higher angular momentum) to one of lower energy, a photon of light energy is emitted whose frequency ν is related to the energy difference ΔE by the equation ν=ΔE/h. For hydrogen, the frequencies of the spectral lines are given by ν=cR (1/nf2−1/ni2) where c is the speed of light, R is the Rydberg constant, and nf and ni are the final and initial quantum numbers of the electron orbits (ni is always greater than nf). The series of spectral lines for which nf=1 is known as the Lyman series; that for nf=2 is the Balmer series; that for nf=3 is the Paschen series; that for nf=4 is the Brackett series; and that for nf=5 is the Pfund series. The Bohr theory was not as successful in explaining the spectra of other substances, but later developments of the quantum theory showed that all aspects of atomic and molecular spectra can be explained quantitatively in terms of energy transitions between different allowed quantum states.

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absorption spectrum

A spectrum that is produced when electromagnetic radiation has been absorbed by matter. Typically, absorption spectra are produced when radiation from an incandescent source, i.e. radiation with a continuous spectrum, passes through cooler matter. Radiation is absorbed (i.e. its intensity is diminished) at selective wavelengths so that a pattern of very narrow dips or of wider troughs – i.e. absorption lines or bands – are superimposed on the continuous spectrum.

The wavelengths at which absorption occurs correspond to the energies required to cause transitions of the absorbing atoms or molecules from lower energy levels to higher levels. In the hydrogen atom, for example, absorption of a photon with the required energy results in a ‘jump’ of the electron from its normal orbit to one of higher energy (see hydrogen spectrum).

The absorption lines (or bands) of a star are produced when elements (or compounds) in the outermost layers of the star absorb radiation from a continuous distribution of wavelengths generated at a lower level in the star. Basically the same elements occur in stars. Since each element has a characteristic pattern of absorption lines for any particular temperature (and pressure) range, there are several different types of stellar spectra depending on the surface temperature of the star. See spectral types. See also emission spectrum.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Absorption Spectrum


the spectrum that results when optical or X-radiation is passed through a substance and is selectively absorbed.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

absorption spectrum

[əb′sȯrp·shən ‚spek·trəm]
A plot of how much radiation a sample absorbs over a range of wavelengths; the spectrum can be a plot of either absorbance or transmittance versus wavelength, frequency, or wavenumber.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
(b) Simulated absorption spectra for W absorber and bare W slab with normally-incident light of x-polarization ([theta] = [phi] = 0, i.e., electric field parallel to the x-axis).
Analyzing the UV-Vis absorption spectra of Figure 4(a) and tests data in Table 2, we can find that the maximum absorption wavelengths of five groups of sample solutions are in the range of 410~430nm, and when [mathematical expression not reproducible], the maximum absorption wavelengths Am has a little small value.
The absorption spectra of all three sensitizers on the Ti[O.sub.2] films exhibited significantly hypsochromic shift and a broader full width at half maximum (fwhm) than absorption spectra measured in CH[Cl.sub.3] due to strong interactions between the dyes and Ti[O.sub.2] surfaces.
Caption: Figure 1: Absorption spectra of surface freshwater from three Karelian lakes, unfiltered (solid lines) and filtered (dashed lines).
Figure 2 shows that the absorption spectra of ASA, DNA, Tm(III), DNA-ASA, DNA-Tm(III), Tm(III)-ASA complex, and Tm(III)-ASA-DNA system at a certain concentration were obtained using a UV-vis spectrometer.
Caption: Figure 6: Near-infrared absorption spectra of ACV ointments, observed to 4000-10000 [cm.sup.-1].
The absorption spectra from 500 nm to 2000 nm at room temperature of 3 at.% Tm, x at.% Y are shown in Figure 2(a).
Huffman, "The infrared and ultraviolet absorption spectra of laboratory produced carbon dust: evidence for the presence of the [C.sub.60] molecule," Chemical Physics Letters, vol.
Absorption spectra of DBM shows broad band with maximum at 343 nm, which is assigned to enolic [pi]-[pi]* transitions of the [beta]-diketone moiety.
In this study, the absorption spectra of PSU (sample 9), PSU-b-PAO-b-PDMS (samples 1-5), and PSU-WAO (sample 6) in 1,2-dichloroethane were recorded at different temperatures and the relative absorbance [A.sub.max]/[A.sup.o] was calculated for each sample (where [A.sub.o] and Amax are the values of maximum absorbance at 20[degrees]C and at a given temperature, t, respectively, when the temperature varies from 25[degrees]C to 75[degrees]C).
Huber, "Coherent potential approximation for the absorption spectra and the densities of states of cubic Frenkel exciton systems with Gaussian diagonal disorder," Physica B: Condensed Matter, vol.
UV-Vis absorption spectra of all synthesized CdSe nanoparticles are shown in Figure 1(a).