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When light passes through a prism spectroscope or a diffraction-grating spectroscope, an optical spectrum is obtained in which the intensity of the radiation may be analyzed as a function of wavelength. The spectrum may be observed visually in the limited wavelength region to which the eye is sensitive; it may be focused on a photographic plate or upon a thermocouple or thermopile. Our knowledge of the structure of atoms and molecules is largely dependent upon the analyses of optical spectra, for these spectra are characteristic of the emitting atoms or molecules. Even before the spectra of atoms were properly understood in terms of the atomic structure, it was possible to determine the chemical composition of an unknown substance by study of its spectrum.
The spectra obtained from radiating bodies are called emission spectra and are classified as continuous spectra, band spectra, and line spectra, according to their appearance in a spectroscope. Continuous spectra are emitted by solids, liquids, and dense opaque gases at high temperatures. The spectrum of the sun, or of a black body, is a continuous spectrum and, as we have seen in Section 18-6, the shape of the black-body spectrum is characteristic of its temperature. Gases at low pressures emit band or line spectra. Line spectra have their origin in the energy changes which take place in the atoms of a gas, while band spectra are associated with similar changes in the molecules.