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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 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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Spectrum

Spectrum

The term spectrum has two different, but closely related, meanings. In general, the term refers to a whole range of things. In everyday life, for example, a person might say that he or she is interested in the whole spectrum of news stories, meaning that he or she enjoys reading and hearing about anything to do with the news.

In the field of science, one meaning for the word spectrum has to do with the whole range of electromagnetic energies that exist. This range is known as the electromagnetic spectrum. All forms of electromagnetic energy travel through space in the form of waves that have distinctive wavelengths and frequencies. The wavelength of a wave is the distance between adjacent identical parts of the wave, as between two crests or two troughs (pronounced trawfs). The frequency of a wave is the number of crests (or troughs) that pass a given point in space per second.

The electromagnetic spectrum consists of forms of energy such as gamma rays, X rays, ultraviolet radiation, infrared radiation, visible light, radio waves, microwaves, and radar. These forms of energy are similar in their mode of transmission but different from each other in their wavelength and frequency.

Words to Know

Absorption spectrum: The spectrum formed when light passes through a cool gas.

Continuous spectrum: A spectrum that consists of every possible wavelength of light or energy.

Electromagnetic spectrum: The continuous distribution of all electromagnetic radiation with wavelengths ranging from approximately 1015 to 106 meters, which includes gamma rays, X rays, ultraviolet, visible light, infrared, microwaves, and radio waves.

Emission spectrum: The spectrum produced when atoms are excited and give off energy.

Frequency: For a wave, the number of crests (or troughs) that pass a stationary point per second.

Line spectrum: A spectrum that consists of a few discrete lines.

Wavelength: The distance between adjacent peaks (peaks located next to each other) or troughs on a wave.

The term spectrum is also used in describing the whole range of visible light, ranging from red through orange, yellow, green, and blue to violet. If all colors are represented in the spectrum, it is called a continuous spectrum. A rainbow is an example of a continuous spectrum.

When any one given element is heated, it also gives off a spectrumbut one that is not continuous. Instead, it gives off a series of lines that reflect specific electron changes that occur within the atoms of that element. Some elements have very simple line spectra consisting of only a handful of lines. Other elements give off more complex line spectra with many lines.

Line spectra can take on one of two general forms: emission or absorption spectra. An emission spectrum is the line pattern formed when an element is excited and gives off energy. An absorption spectrum is formed when white light passes through a cool gas. The gas absorbs certain wavelengths of energy and allows others to pass through. The line spectrum formed by the energy that passes through the gas is known as an absorption spectrum.

[See also Electromagnetic spectrum; Light; Spectroscopy ]

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spectrum

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.

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spectrum

spectrum Arrangement of electromagnetic radiations ordered by wavelength or frequency. The visible light spectrum is a series of colours: red, orange, yellow, green, blue, indigo, and violet. Each colour corresponds to a different wavelength of light. This was first noted in 1666 by English physicist Isaac Newton. A spectrum is seen in a rainbow or when white light passes through a prism. This effect, also seen when visible light passes through a diffraction grating, produces a continuous spectrum in which all wavelengths (between certain limits) are present. Spectra formed from objects emitting radiations are called emission spectra. These occur when a substance is strongly heated or bombarded by electrons. An absorption spectrum, consisting of dark regions on a bright background, is obtained when white light passes through a semi-transparent medium that absorbs certain frequencies. A line spectrum is one in which only certain wavelengths or ‘lines’ appear. See spectroscopy

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spectrum

spec·trum / ˈspektrəm/ • n. (pl. -tra / -trə/ ) 1. a band of colors, as seen in a rainbow, produced by separation of the components of light by their different degrees of refraction according to wavelength. ∎  (the spectrum) the entire range of wavelengths of electromagnetic radiation. ∎  an image or distribution of components of any electromagnetic radiation arranged in a progressive series according to wavelength. ∎  a similar image or distribution of components of sound, particles, etc., arranged according to such characteristics as frequency, charge, and energy. 2. used to classify something, or suggest that it can be classified, in terms of its position on a scale between two extreme or opposite points: the left or the right of the political spectrum. ∎  a wide range: self-help books are covering a broader and broader spectrum.

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spectrum

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.

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spectrum

spectrum (spek-trŭm) n. (in pharmacology) the range of effectiveness of an antibiotic. broad s. effectiveness against a wide range of microoganisms.

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spectrum

spectrum •minimum • maximum • optimum •chrysanthemum, helianthemum •cardamom • Pergamum • sesamum •per annum • magnum • damnum •Arnhem, Barnum •envenom, venom •interregnum • Cheltenham • arcanum •duodenum, plenum •platinum • antirrhinum • Bonham •summum bonum • Puttnam •ladanum • molybdenum • laudanum •origanum, polygonum •organum • tympanum •laburnum, sternum •gingham • Gillingham • Birmingham •Cunningham • Walsingham •Nottingham • wampum • carom •Abram • panjandrum • tantrum •angstrom • alarum • candelabrum •plectrum, spectrum •arum, harem, harum-scarum, Sarum •sacrum, simulacrum •maelstrom • cerebrum • pyrethrum •Ingram •sistrum, Tristram •Hiram •grogram, pogrom •nostrum, rostrum •cockalorum, decorum, forum, jorum, Karakoram, Karakorum, Mizoram, pons asinorum, quorum •wolfram • fulcrum • Durham •conundrum • buckram • lustrum •serum, theorem •labarum • marjoram • pittosporum •Rotherham • Bertram

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Spectrum

Spectrum

The spectrum of light

The wave nature of light

The electromagnetic spectrum

Emission spectra

Absorption spectra

Resources

Certain properties of objects or physical processes, such as the frequency of light or sound, the masses of the component parts of a molecule, or even the ideals of a political party, may have a wide variety of values. The distribution of these values, arranged in increasing or decreasing order, is the spectrum of that property. For example, sunlight is made up of many different colors of light, the full spectrum of which are revealed when sunlight is dispersed, as it is in a rainbow. Similarly, the distribution of sounds over a range of frequencies, such as a musical scale, is a sound spectrum. The masses of fragments from an ionized molecule, separated according to their mass-to-charge ratio, constitute a mass spectrum. Opposing political parties are often said to be on opposite ends of the (political) spectrum. The term spectrum is also used to describe the graphical illustration of a spectrum of values. The plural of spectrum is spectra.

The spectrum of light

The spectrum of colors contained in sunlight was discovered by Sir Isaac Newton in 1666. In fact, the word spectrum was coined by Newton to describe the phenomenon he observed. In a report of his discovery published in 1672, Newton described his experiment as follows:

I procured me a triangular glass prism, having darkened my chamber and made a small hole in my window shuts, to let in a convenient quantity of the suns light, I placed my prism at this entrance, that it might be thereby refracted to the opposite wall. It was at first a pleasing divertissement to view the vivid and intense colours produced thereby. A diagram of Newtons experiment is illustrated in Figure 1. Newton divided the spectrum of colors he observed into the familiar sequence of seven fundamental colors: red, orange, yellow, green, blue, indigo, violet (ROYGBIV). He chose to divide the spectrum into seven colors in analogy with the seven fundamental notes of the musical scale. However, both divisions are completely arbitrary as the sound and light spectrum each contain a continuous distribution (and therefore an infinite number) of colors and notes.

The wave nature of light

Light can be pictured as traveling in the form of a wave. A wave is a series of regularly spaced peaks and troughs. The distance between adjacent peaks (or troughs) is the wavelength, symbolized by the Greek

letter lambda (λ). For a light wave traveling at a speed, c, the number of peaks (or troughs) which pass a stationary point each second is the frequency of the wave, symbolized by the Greek letter nu (υ). The units of frequency are number per second, termed Hertz (Hz). The frequency of a wave is related to the wavelength and the speed of the wave by the simple relation: υ = c/λ. The speed of light depends on the medium through which it is passing, but, as light travels primarily only through air or space, its speed may be considered to be constant, with a value of 3.0 × 108meters/sec. Therefore, since c is a constant, light waves may be described by either their frequency or their wavelength, which can be interconverted through the relation υ = c/λ.

Interestingly, Newton did not think light traveled as a wave, but rather believed light to be a stream of particles, which he termed corpuscles, emitted by the light source and seen when they physically entered the eye. It was Newtons contemporary, the Dutch astronomer Christiaan Huygens (16291695), who first theorized that light traveled from the source as a series of waves. In the quantum mechanical description of light, the basic tenets of which were developed in the early 1900s by Max Planck and Albert Einstein, light is considered to possess both particle and wave characteristics. A particle of light is called a photon, and can be thought of as a bundle of energy emitted by the light source. The energy carried by a photon of light, E, is equal to the frequency of the light, υ, multiplied by a constant: E = hν, where h is Plancks constant (h = 6.626 × 1034 joules-seconds), named in honor of Max Planck. Thus, according to the quantum mechanical theory of light, light traveling through air or space may be described by any one of three interrelated quantities: frequency, wavelength, or energy. A spectrum of light may therefore be represented as a distribution of intensity as a function of any (or all) of these measurable quantities.

The electromagnetic spectrum

Light is a form of electromagnetic radiation. Electromagnetic waves travel at the speed of light and can have almost any frequency or wavelength. The distribution of electromagnetic radiation according to its frequency or wavelength (or energy) is the electromagnetic spectrum. The electromagnetic spectrum is the continuous distribution of frequencies of electromagnetic radiation ranging from approximately 105 Hz (radio waves) up to greater than 1020Hz (x-rays and gamma rays). Equivalently, it is the distribution of wavelengths of electromagnetic radiation ranging from very long (λ = 106 meters, radio waves) to the very short wavelengths of x-rays and gamma rays (λ = 10-15 meters). Note that the higher frequencies correspond to lower wavelengths and vice versa (υ = c/λ). Finally, the electromagnetic spectrum can also be separated according to the photon energy of the radiation, ranging from 10-29 joules (radio waves) up to 1014 Joules (x-rays and gamma rays). Note that photon energy increases with increasing frequency (E=hν ).

The electromagnetic spectrum can be divided into regions that exhibit similar properties, each of which itself constitutes a spectrum: the x-ray spectrum, the ultraviolet spectrum, the visible spectrum (which we commonly refer to as light), the infrared spectrum and the radio-frequency spectrum. However, these divisions are arbitrary and do not imply a sharp change in the character of the radiation. The visible light spectrum, while comprising only a small portion of the entire electromagneticspectrum, can be further divided into the colors of the rainbow as was demonstrated by Newton. The other regions of the electro-magneticspectrum, although invisible to our eyes, are familiar to us through other means: x rays expose x-ray sensitive film, ultraviolet light causes sunburn, microwaves heat food, and radio frequency waves carry radio and television signals.

The interaction of electromagnetic radiation with matter is studied in the field of spectroscopy. In this field, spectra are used as a means to graphically illustrate which frequencies, wavelengths, or photon energies of electromagnetic radiation interact the strongest with the material under investigation. These spectra are usually named according to the spectroscopic method used in their generation: nuclear magnetic resonance (NMR) spectroscopy generates NMR spectra, microwave spectroscopy generates microwave spectra, and so forth. In addition, these spectra may also be named according to the origin or final fate of the radiation (emission spectrum, absorption spectrum), the nature of the material under study (atomic spectrum, molecular spectrum) and the width of the electromagnetic spectrum that undergoes the interaction (discrete, line, continuous, or band spectrum).

Emission spectra

The spectrum of electromagnetic radiation emitted by a source is an emission spectrum. One way of producing electromagnetic radiation is by heating a material until it glows, or emits light. For example, a piece of iron heated in a blacksmiths furnace will emit visible light as well as infrared radiation (heat). Similarly, a light bulb uses electrical current to heat a tungsten filament encased in an evacuated glass bulb. The Sun is a source of radiation in the infrared, visible, and ultraviolet regionsof the electromagnetic spectrum. Radiation

produced by a thermal source is called black body, or incandescent radiation (Figure 2). Spectra such as these, in which the intensity varies smoothly over the distribution range, are called continuous spectra.

Atoms that have been heated (such as by an electric spark or a flame) will also emit electromagnetic radiation. However, if there are only a few atoms present so that they do not collide with one another, such as in a low-pressure gas, the excited atoms will emit radiation at only a few specific wavelengths. For example, a vapor of neon atoms in a glass tube excited by an electrical discharge produces the familiar red color of neon lights by emitting light of only red wavelengths. In contrast to continuous spectra, atomic emission spectra generally exhibit high intensity at

KEY TERMS

Absorption spectrum The record of wavelengths (or frequencies) of electromagnetic radiation absorbed by a substance; the absorption spectrum of each pure substance is unique.

Band spectrum A spectrum in which the distribution of values of the measured property occurs in distinct groups. In an absorption spectrum, the absorbed wavelengths (or frequencies) occur in broad, but distinct, groups. Band spectra are usually associated with molecular absorbers.

Continuous spectrum A spectrum in which there are no breaks in the distribution of values associated with the measured property.

Electromagnetic spectrum The continuous distribution of all electromagnetic radiation with wavelengths ranging from approximately 1015 to 106 meters which includes: gamma rays, x rays, ultraviolet, visible light, infrared, microwaves, and radio waves.

Emission spectrum The record of wavelengths (or frequencies) of electromagnetic radiation emitted by a substance which has previously absorbed energy, typically from a spark or a flame. The emission spectrum of each pure substance is unique.

Frequency For a traveling wave, the number of wavelengths that pass a stationary point per unit of time, usually expressed in #/sec, or Hertz (Hz), and symbolized by υ.

Line spectrum A spectrum, usually associated with isolated atomic absorbers or emitters, in which only a few discrete values of the measured property occur. Line spectra are also called discrete spectra.

Wavelength The distance between two consecutive crests or troughs in a wave.

only a few wavelengths and very low intensity at all others; such discontinuous spectra are called discrete spectra.

Absorption spectra

Atomic and molecular materials can also absorb electromagnetic radiation. The set of wavelengths or frequencies of electromagnetic radiation absorbed by any single, pure material is unique to that material, and can be used as a fingerprint to identify the material. The record of the absorbed wavelengths or frequencies is an absorption spectrum.

The instrument used to measure the absorption spectrum of a material is called a spectrometer. Newtons experiment, illustrated in Figure 1, has all but one of the components of a simple absorption spectrometer: a sample placed between the light source and the prism. With a sample in place, some of the wavelengths of sunlight (consisting of all visible wavelengths) will be absorbed by the sample. Light not absorbed by the sample will, as before, be separated (dispersed) into its component wavelengths (colors) by the prism. The appearance of the spectrum will resemble that obtained without the sample in place, with the exception that those wavelengths that have been absorbed are missing, and will appear as dark lines within the spectrum of colors. If a piece of the photographic film is used instead of the card, the absorption spectrum can be recorded.

The absorption spectrum of gaseous hydrogen atoms recorded on a photographic plate is presented in Figure 3. Atomic spectra recorded on photographic plates were among the earliest to be studied, and the appearance of these spectra led to the use of the term line spectrum to describe atomic spectra (either emission or absorption). The term is still commonly used even if the spectra are not recorded photographically.

Molecules also absorb electromagnetic radiation, but in contrast to atoms, molecules will absorb broader regions, or bands, of the electromagnetic spectrum. Molecular spectra are therefore often referred to as band spectra.

See also Blackbody radiation; Spectral lines.

Resources

BOOKS

Baykal, Altan, et al. The Electromagnetic Spectrum of Neutron Stars. New York: Springer, 2005.

Cheshire, Gerard. Light and Colors. New York: Smart Apple Media, 2006.

Nassau, K. The Physics and Chemistry of Color. New York: John Wiley and Sons, 1983.

PERIODICALS

Walker, J. The Amature Scientist: The Spectra of Streetlights Illuminate Basic Principles of Quantum Mechanics. Scientific American 250 (January 1984): 138-42.

Karen Trentelman

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Spectrum

Spectrum

Certain properties of objects or physical processes, such as the frequency of light or sound, the masses of the component parts of a molecule , or even the ideals of a political party, may have a wide variety of values. The distribution of these values, arranged in increasing or decreasing order, is the spectrum of that property. For example, sunlight is made up of many different colors of light, the full spectrum of which are revealed when sunlight is dispersed, as it is in a rainbow. Similarly, the distribution of sounds over a range of frequencies, such as a musical scale, is a sound spectrum. The masses of fragments from an ionized molecule, separated according to their mass-to-charge ratio , constitute a mass spectrum. Opposing political parties are often said to be on opposite ends of the (political) spectrum. The term spectrum is also used to describe the graphical illustration of a spectrum of values. The plural of spectrum is spectra.

The spectrum of light

The spectrum of colors contained in sunlight was discovered by Sir Isaac Newton in 1666. In fact, the word "spectrum" was coined by Newton to describe the phenomenon he observed. In a report of his discovery published in 1672, Newton described his experiment as follows:

"I procured me a triangular glass prism,... having darkened my chamber and made a small hole in my window shuts, to let in a convenient quantity of the sun's light, I placed my prism at this entrance, that it might be thereby refracted to the opposite wall. It was at first a pleasing divertissement to view the vivid and intense colours produced thereby." A diagram of Newton's experiment is illustrated here. Newton divided the spectrum of colors he observed into the familiar sequence of seven fundamental colors: red, orange, yellow, green, blue, indigo, violet (ROYGBIV). He chose to divide the spectrum into seven colors in analogy with the seven fundamental notes of the musical scale. However, both divisions are completely arbitrary as the sound and light spectrum each contain a continuous distribution (and therefore an infinite number) of "colors" and "notes."

The wave nature of light

Light can be pictured as traveling in the form of a wave. A wave is a series of regularly spaced peaks and troughs. The distance between adjacent peaks (or troughs) is the wavelength, symbolized by the Greek letter lambda ( λ). For a light wave traveling at a speed, c, the number of peaks (or troughs) which pass a stationary point each second is the frequency of the wave, symbolized by the Greek letter nu ( ν). The units of frequency are number per second, termed Hertz (Hz). The frequency of a wave is related to the wavelength and the speed of the wave by the simple relation : ν = c/ λ. The speed of light depends on the medium through which it is passing, but, as light travels primarily only through air or space , its speed may be considered to be constant, with a value of 3.0 × 108 meters/sec. Therefore, since c is a constant, light waves may be described by either their frequency or their wavelength, which can be interconverted through the relation ν = c/ λ.

Interestingly, Newton did not think light traveled as a wave, but rather he believed light to be a stream of particles, which he termed corpuscles, emitted by the light source and seen when they physically entered the eye . It was Newton's contemporary, the Dutch astronomer Christiaan Huygens (1629-1695), who first theorized that light traveled from the source as a series of waves. In the quantum mechanical description of light, the basic tenets of which were developed in the early 1900s by Max Planck and Albert Einstein, light is considered to possess both particle and wave characteristics. A "particle" of light is called a photon , and can be thought of as a bundle of energy emitted by the light source. The energy carried by a photon of light, E, is equal to the frequency of the light, ν , multiplied by a constant: E = h ν, where h is Planck's constant, (h = 6.626 × 10-34 joulesseconds), named in honor of Max Planck. Thus, according to the quantum mechanical theory of light, light traveling through air or space may be described by any one of three inter-related quantities: frequency, wavelength, or energy. A spectrum of light may therefore be represented as a distribution of intensity as a function of any (or all) of these measurable quantities.


The electromagnetic spectrum

Light is a form of electromagnetic radiation . Electromagnetic waves travel at the speed of light and can have almost any frequency or wavelength. The distribution of electromagnetic radiation according to its frequency or wavelength (or energy) is the electromagnetic spectrum. The electromagnetic spectrum is the continuous distribution of frequencies of electromagnetic radiation ranging from approximately 105 Hz (radio waves ) up to greater than 1020 Hz (x-rays and gamma rays). Equivalently, it is the distribution of wavelengths of electromagnetic radiation ranging from very long ( λ = 106 meters, radio waves) to the very short wavelengths of x-rays and gamma rays ( λ = 10-15 meters). Note that the higher frequencies correspond to lower wavelengths and vice versa ( ν = c/ λ). Finally, the electromagnetic spectrum can also be separated according to the photon energy of the radiation, ranging from 10-29 joules (radio waves) up to 10-14 joules (x-rays and gamma rays). Note that photon energy increases with increasing frequency (E=h ν).

The electromagnetic spectrum can be divided into regions which exhibit similar properties, each of which itself constitutes a spectrum: the x-ray spectrum, the ultraviolet spectrum, the visible spectrum (which we commonly refer to as "light"), the infrared spectrum and the radio-frequency spectrum. However, these divisions are arbitrary and do not imply a sharp change in the character of the radiation. The visible light spectrum, while comprising only a small portion of the entire electromagneticspectrum, can be further divided into the colors of the rainbow as was demonstrated by Newton. The other regions of the electromagneticspectrum, although invisible to our eyes, are familiar to us through other means: x rays expose x-ray sensitive film, ultraviolet light causes sunburn, microwaves heat food, and radio frequency waves carry radio and television signals.

The interaction of electromagnetic radiation with matter is studied in the field of spectroscopy . In this field, spectra are used as a means to graphically illustrate which frequencies, wavelengths, or photon energies of electromagnetic radiation interact the strongest with the material under investigation. These spectra are usually named according to the spectroscopic method used in their generation: nuclear magnetic resonance (NMR) spectroscopy generates NMR spectra, microwave spectroscopy generates microwave spectra, and so forth. In addition, these spectra may also be named according to the origin or final fate of the radiation (emission spectrum, absorption spectrum), the nature of the material under study (atomic spectrum, molecular spectrum) and the width of the electromagnetic spectrum which undergoes the interaction (discrete, line, continuous, or band spectrum).

Emission spectra

The spectrum of electromagnetic radiation emitted by a source is an emission spectrum. One way of producing electromagnetic radiation is by heating a material until it glows, or emits light. For example, a piece of iron heated in a blacksmith's furnace will emit visible light as well as infrared radiation (heat). Similarly, a light bulb uses electrical current to heat a tungsten filament encased in an evacuated glass bulb. The Sun is a source of radiation in the infrared, visible and ultraviolet regions of the electromagnetic spectrum. Radiation produced by a thermal source is called black body, or incandescent radiation. Spectra such as these, in which the intensity varies smoothly over the distribution range, are called continuous spectra.


Atoms that have been heated (typically by a high-energy source such as an electric spark or a flame), will also emit electromagnetic radiation. However, if there are only a few atoms present so that they do not collide with one another, such as in a low-pressure gas, the excited atoms will emit radiation at only a few specific wavelengths. For example, a vapor of neon atoms in a glass tube excited by an electrical discharge produces the familiar red color of neon lights by emitting light of only red wavelengths. In contrast to a continuous spectrum, atomic emission spectra generally exhibit high intensity at only a few wavelengths and very low intensity at all others; such discontinuous spectra are called discrete spectra.

Absorption spectra

Atomic and molecular materials can also absorb electromagnetic radiation. The set of wavelengths or frequencies of electromagnetic radiation absorbed by any single, pure material is unique to that material, and can be used as a "fingerprint" to identify the material. The record of the absorbed wavelengths or frequencies is an absorption spectrum.

The instrument used to measure the absorption spectrum of a material is called a spectrometer. Newton's experiment, illustrated in Figure 1, has all but one of the components of a simple absorption spectrometer: a sample placed between the light source and the prism. With a sample in place, some of the wavelengths of sunlight (consisting of all visible wavelengths) will be absorbed by the sample. Light not absorbed by the sample will, as before, be separated (dispersed) into its component wavelengths (colors) by the prism. The appearance of the spectrum will resemble that obtained without the sample in place, with the exception that those wavelengths which have been absorbed are missing, and will appear as dark lines within the spectrum of colors. If a piece of the photographic film is used instead of the card, the absorption spectrum can be recorded.

The absorption spectrum of gaseous hydrogen atoms recorded on a photographic plate is presented here. Atomic spectra recorded on photographic plates were among the earliest to be studied, and the appearance of these spectra led to the use of the term "line spectrum" to describe atomic spectra (either emission or absorption). The term is still commonly used even if the spectra are not recorded photographically.

Molecules also absorb electromagnetic radiation, but in contrast to atoms, molecules will absorb broader regions, or bands, of the electromagnetic spectrum. Molecular spectra are therefore often referred to as band spectra.

See also Blackbody radiation; Spectral lines.


Resources

books

Lide, D.R., ed. CRC Handbook of Chemistry and Physics. Boca Raton: CRC Press, 2001.

Nassau, K. The Physics and Chemistry of Color. New York: John Wiley and Sons, 1983.


periodicals

Walker, J. "The Amature Scientist: The Spectra of Streetlights Illuminate Basic Principles of Quantum Mechanics." Scientific American 250 (January 1984): 138-42.


Karen Trentelman

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Absorption spectrum

—The record of wavelengths (or frequencies) of electromagnetic radiation absorbed by a substance; the absorption spectrum of each pure substance is unique.

Band spectrum

—A spectrum in which the distribution of values of the measured property occurs in distinct groups. In an absorption spectrum, the absorbed wavelengths (or frequencies) occur in broad, but distinct, groups. Band spectra are usually associated with molecular absorbers.

Continuous spectrum

—A spectrum in which there are no breaks in the distribution of values associated with the measured property.

Electromagnetic spectrum

—The continuous distribution of all electromagnetic radiation with wavelengths ranging from approximately 1015 to 106 meters which includes: gamma rays, x rays, ultraviolet, visible light, infrared, microwaves, and radio waves.

Emission spectrum

—The record of wavelengths (or frequencies) of electromagnetic radiation emitted by a substance which has previously absorbed energy, typically from a spark or a flame. The emission spectrum of each pure substance is unique.

Frequency

—For a traveling wave, the number of wavelengths that pass a stationary point per unit of time, usually expressed in #/sec, or Hertz (Hz), and symbolized by ν.

Line spectrum

—A spectrum, usually associated with isolated atomic absorbers or emitters, in which only a few discrete values of the measured property occur. Line spectra are also called discrete spectra.

Wavelength

—The distance between two consecutive crests or troughs in a wave.

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