Spectrometer

Spectrometer, pronounced spehk TRAHM uh tuhr, is an instrument that spreads out light into a spectrum and displays it for study. The atoms or molecules of all substances give off light when heated to high temperatures. The pattern of light given off is different for every substance. No two substances have the same spectrum. Thus, experts can identify a substance or determine its chemical composition by analyzing its spectrum.

Spectrometers are used to examine a wide range of materials. Industrial chemists use these instruments to detect impurities in steel and other metal alloys. Spectrometers enable astronomers to study the chemical composition of the stars. Spectrometers are also used to identify chemical substances found at the scene of a crime and to detect pollutants in the air and water.

A typical spectrometer is enclosed by a container that keeps out light not being studied. Light enters through the narrow entrance slit and passes through a collimating lens. This lens causes the light to become a beam of parallel light rays. The parallel light then travels through a prism, where it is broken up into a spectrum. A lens focuses the light on the exit slit. Only one color of light can pass through this slit at a time. Therefore, the prism must be rotated to bring the other colors into the exit slit and to scan the entire spectrum. A circular scale records the angle of the prism, from which the wavelength of the light can be determined.

Some spectrometers have a flat mirror called a grating, instead of a prism. The surface of a grating is lined with thousands of narrow, parallel grooves. Upon striking a grating, a parallel beam of light spreads out into a spectrum. See Diffraction.

There are several kinds of spectrometers. A spectroscope has a telescope for visual observation of a spectrum. A spectrograph photographs a spectrum by recording its image on a photographic plate. A spectrophotometer scans a spectrum and measures the brightness of each of its colors. Many spectrophotometers have a photomultiplier tube that produces an electric current proportional in strength to the brightness of the light that is being measured.

Contributor: Sandra M. Faber, Ph.D., Prof. of Astronomy, Lick Observatory, Univ. of California, Santa Cruz.

Spectrometer

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