Spectroscopy, the study of matter and its properties by the use of spectra. Each chemical element, when heated so that it gives off light, yields a different series of lines in a spectrum. The position of the lines and the patterns they form in the spectrum of a particular substance can yield many kinds of information about the substance. Spectroscopy is not limited to the visible-light spectrum, but includes the spectra of other forms of electromagnetic radiation, such as X rays, infrared radiation, and radio waves.

Spectroscopy is extensively used in physics, biology, chemistry, and industry to determine the composition and purity of substances. Astronomers use spectroscopy to learn the temperature, composition, and motion of celestial objects from the radiation the objects emit. Spectroscopy has played a vital role in the development of modern theories about the atom.

The basic instrument in spectroscopy is the spectroscope, a device that receives and disperses incoming light or other radiant energy into a spectrum. The method of dispersion is usually either diffraction or refraction. A spectroscope that produces a record of a spectrum is called a spectrograph.

With a typical spectroscope, rays of light enter through a collimator, a tube with a narrow slit at one end and a lens at the other. Together, the slit and lens make the incoming rays parallel to one another. From the collimator, the waves pass through a prism or a diffraction grating, which separates the waves by wavelength, forming a spectrum. Segments of the spectrum can be viewed directly with a small telescope mounted on the spectroscope. A spectrograph records an image on a photographic plate or may be filled with some other recording device such as a photoelectric detector.

Spectroscopes vary considerably in design, depending on the substances to be studied, the kinds of information desired, and the method of receiving and recording the information. Detailed information about the sun has been obtained with the spectroheliograph, a spectroscope designed for obtaining an image of the sun at one or more chosen wavelengths. Spectroscopes that measure the position of spectral lines or the intensity of a spectrum at given wavelengths are called spectrometers. Various types of spectrometers are sensitive to radiation at wavelengths outside the visible light spectrum. Instruments called interferometers can be used as high-resolution spectrometers.