spectroscopy (spektroskopi)

[As spectro scope + -y3.] That branch of science, more particularly of chemical and physical science, which is concerned with the use of the spectroscope and with spectrum analysis. spectrum (spektrum), n.; pl. spectra (-trä). [ 5811 ing spot will be green; if black, it will be changed into white. These images are also termed ocular spectra. 3. In physics, the continuous band of light ( vis i ble spectrum) showing the succe ive prismatic colors, or the isolated lines or bands of color, observed when the radiation from such a source as the sun, or an ignited vapor in a gas-flame, is viewed after having been pa ed through a prism ( prismatic spectrum ) or reflected from a diffraction-grating ( diffraction- or interference spectrum). The action of the prism (see prism and refraction) is to refract the light and at the same time to separate or disperse the rays of different wave-lengths, the refraction and dispersion being greater as the wavelength diminishes. The grating (see grating2, 2), which consists usually of a series of fine parallel lines (say 10,000 or 20,000 to the inch) ruled on speculum-metal, diffracts and at the same time disperses the light-rays, forming a series of spectra whose lengths depend upon the finene of the lines. If, now, a beam of white light is pa ed through a slit, and then by a collimator lens is thrown upon a prism, and the light from this received upon a screen, a colored band will be obtained pa ing by insensible degrees, from the le refrangible end, the red, to the more refrangible end, the violet, through a series of colors ordinarily described as red, orange, yellow, green, blue, indigo, and violet. A similar effect is obtained from a grating, with, however, this difference, that in the prismatic spectrum the red covers only a small part relatively of the colored band, since the action of the prism is to crowd together the le refrangible rays and separate the more refrangible rays of le wave-length, and thus distort the spectrum. The diffraction-spectrum, on the other hand, shows the red occupying about the same space as the blue and violet, and is called a normal spectrum. When the light from different sources is studied in the spectroscope, it is found, first, that a solid or a liquid when incandescent gives a continuous spectrum, and this is true of gases also at great pre ures; second, bodies in the gaseous form give discontinuous spectra, consisting of colored bright lines ( line - spectrum ) or bands ( band - speс trum), or of bands which under certain conditions appear as channeled spaces or flutings ( fluted spectrum ), and these lines or bands for a given substance have a definite position, and are hence characteristic of it; third, if light from an incandescent solid or liquid body pa es through a gas (at a lower temperature than the incandescent body), the gas absorbs the same rays as those its own spectrum consists of; therefore, in this case, the result is a spectrum (absorption- spectrum ) continuous, except as interrupted by black lines occupying the same position as the bright lines in the spectrum of the gas itself would occupy. An absorption-spectrum, showing more or le sharply defined dark bands, is also obtained when the light has pa ed through an appropriate liquid (as blood), or a solid such as a salt of didymium (see further under absorption). For example, the spectrum from a candle-flame is continuous, being due to the incandescent carbon particles suspended in the flame. If, however, the yellow flame produced when a little sodium is inserted in the non-luminous flame of a Bunsen burner is examined, a bright-yellow line is observed; if a red lithium flame, then a red and a yellow line are seen; the red strontium flame gives a more complex spectrum, consisting of a number of lines, chiefly in the red and yellow; and so of other similar substances. For substances like iron, and other metals not volatile except at very high temperatures, the heat of the voltaic arc is employed, and by this means their spectra, often consisting of a hundred or more lines (of iron at least 2,000), can be mapped out. Still again, if the light from the sun is studied in the same way, it is found to be a bright spectrum from red to violet, but cro ed by a large number of dark lines called Fraunhofer lines, because, though earlier seen by Wollaston (1802), they were first mapped by Fraunhofer in 1814; this name is given especially to the more prominent of them, which he designated by the IMG:content-1682.png:[blocks in formation] letters A to H, etc. (See the figures.) These lines, as explained above, are due to the absorption by gases, either in the sun's atmosphere or in that of the earth. When the light is pa ed through a train of prisms, or reflected from a Rowland grating, and thus a very high degree of dispersion obtained, the rays are more widely separated and the spectrum can be more ore minutely examined. Studied in this way, it is found that the dark lines in the solar spec trum number many you sands, the greater part of which can be identified in the spectra of known terrestrial substances. Thus, the presence in the sun's atmosphere of thirty-six elements has been established (Rowland, 1891); these include sodium, pota ium, calcium, magnesium, iron, copper, cobalt, silver, lead, tin, zinc, titanium, aluminium, chromium, silicon, carbon, hydrogen, etc. The radiation from the sun consists not only of those rays whose wave-length is such as to produce the effect of vision upon the eye, but also of others of greater wavelength than the red rays and le wave-length than the violet; the spectrum from such a source consequently includes, besides the luminous part, an invisible part ( in visible spectrum ) below the red, called the infra -red region, and another beyond the violet, called the ultra specular See