spectrum

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 Δ E by the equation ν=Δ E / h. For hydrogen, the frequencies of the spectral lines are given by ν= cR (1/ n_f² -1/ n_i² ) where c is the speed of light, R is the Rydberg constant , and n_f and n_i are the final and initial quantum numbers of the electron orbits ( n_i is always greater than n_f ). The series of spectral lines for which n_f =1 is known as the Lyman series; that for n_f =2 is the Balmer series; that for n_f =3 is the Paschen series; that for n_f =4 is the Brackett series; and that for n_f =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.

spectrum (pl. spectra; optical emission spectrum) A series of lines (line spectra), produced as electrons return to their original energy levels and emit excess energy as infrared, visible, or ultraviolet light of characteristic wavelengths, after atoms have been heated strongly and valence electrons in the outer shell have moved to higher energy levels. Each element has a characteristic line spectrum. The intensity of each line is related to the concentration of the element being excited.

spectrum (pl. spectra) A range of electromagnetic energies arrayed in order of increasing or decreasing wavelength or frequency. The emission spectrum of a body or substance is the characteristic range of radiations it emits when it is heated, bombarded by electrons or ions, or absorbs photons. The absorption spectrum of a substance is produced by examining, through the substance and through a spectroscope, a continuous spectrum of radiation. The energies removed from the continuous spectrum by the absorbing medium show up as black lines or bands; with a substance capable of emitting a spectrum these are in exactly the same positions in the spectrum as the emission lines and bands would occur in the emission spectrum.

Emission and absorption spectra may show a continuous spectrum, a line spectrum, or a band spectrum. A continuous spectrum contains an unbroken sequence of frequencies over a relatively wide range; it is produced by incandescent solids, liquids, and compressed gases. Line spectra are discontinuous lines produced by excited atoms and ions as they fall back to a lower energy level. Band spectra (closely grouped bands of lines) are characteristic of molecular gases or chemical compounds. Absorption spectra of chlorophylls and other photosynthetic pigments are important in the study of photosynthesis. See action spectrum.