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Spectrum

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.

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


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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.

Additional topics

Science EncyclopediaScience & Philosophy: Spectroscopy to Stoma (pl. stomata)Spectrum - The Spectrum Of Light, The Wave Nature Of Light, The Electromagnetic Spectrum, Emission Spectra