2.5 Spectroscopy

Radiation can be analyzed with an instrument known as a spectroscope. In its most basic form, this device consists of an opaque barrier with a slit in it (to form a narrow beam of light), a prism (to split the beam into its component colors), and either a detector or a screen (to allow the user to view the resulting spectrum). Figure 2.11 shows such an arrangement.

Figure 2.11 Spectroscope Diagram of a simple spectroscope. A slit in the barrier at left allows a narrow beam of light to pass. The beam passes through a prism and is split into its component colors. A lens then focuses the light into a sharp image that is either projected onto a screen, as shown here, or analyzed as it is passed through a detector.

Emission Lines

The spectra encountered in the previous section are examples of continuous spectra. A lightbulb, for instance, emits radiation of all wavelengths (mostly in the visible and near-infrared ranges), with an intensity distribution that is well described by the blackbody curve corresponding to the bulb’s temperature. Viewed through a spectroscope, the spectrum of the light from the bulb would show the familiar rainbow running from red to violet without interruption, as presented in Figure 2.11.

Figure 2.12 Emission Spectrum Instead of a continuous spectrum, the light from excited (heated) gydrogen gas consists of a series of distinct spectral lines. (For simplicity, the focusing lenses have been omitted.)

Not all spectra are continuous, however. For instance, if we took a glass jar containing pure hydrogen gas and passed an electrical discharge through it (a little like a lightning bolt arcing through Earth’s atmosphere), the gas would begin to glow—that is, it would emit radiation. If we were to examine that radiation with our spectroscope, we would find that its spectrum consisted of only a few bright lines on an otherwise dark background, quite unlike the continuous spectrum described for the lightbulb. Figure 2.12 shows this schematically (the lenses have been removed for clarity), and a more detailed rendering of the spectrum of hydrogen appears in the top panel of Figure 2.13. The light produced by the hydrogen in this experiment does not consist of all possible colors but instead includes only a few narrow, well-defined emission lines, narrow “slices” of the continuous spectrum. The black background represents all the wavelengths not emitted by hydrogen.

After some experimentation, we would also find that although we could alter the intensity of the lines (for example, by changing the amount of hydrogen in the jar or the strength of the electrical discharge), we could not alter their color (in other words, their frequency or wavelength). This particular pattern of spectral emission lines is a property of the element hydrogen—whenever we perform this experiment, the same characteristic emission spectrum is the result. Other elements yield different emission spectra. Depending on which element is involved, the pattern of lines can be fairly simple or very complex. Always, though, it is unique to that element. The emission spectrum of a gas thus provides a “fingerprint” that allows scientists to deduce its presence by spectroscopic means. Examples of the emission spectra of some common substances are shown in Figure 2.13.

Figure 2.13 Elemental Emission The emission spectra of some well-known elements. (Wabash Instrument Corporation)

Absorption Lines

When sunlight is split by a prism, at first glance it appears to produce a continuous spectrum. However, closer scrutiny shows that the solar spectrum is interrupted by a large number of narrow dark lines, as shown in Figure 2.14. These lines represent wavelengths of light that have been removed (absorbed) by gases present either in the outer layers of the Sun or in Earth’s atmosphere. These gaps in the spectrum are called absorption lines. The absorption lines in the solar spectrum are referred to collectively as Fraunhofer lines, after the nineteenth-century German physicist Joseph Fraunhofer, who measured and catalogued more than 600 of them.

Figure 2.14 Solar Spectrum This visible spectrum of the Sun shows hundreds of dark absorption lines superimposed on a bright continuous spectrum. Here, the scale extends from long wavelengths (red) at the upper left to short wavelengths (blue) at the lower right. (AURA)

At around the time solar absorption lines were discovered, scientists found that absorption lines could also be produced in the laboratory by passing a beam of light from a continuous source through a cool gas, as shown in Figure 2.15. They quickly observed an intriguing connection between emission and absorption lines: The absorption lines associated with a given gas occur at precisely the same wavelengths as the emission lines produced when the gas is heated. Both sets of lines therefore contain the same information about the composition of the gas.

Figure 2.15 Absorption Spectrum When a cool gas is placed between a source of continuous radiation (a lightbulb) and the detector/screen, the resulting color spectrum is crossed by a series of dark absorption lines. These lines are formed when the cool gas absorbs certain wavelengths (colors) from the original beam of light. The absorption lines appear at precisely the same wavelengths as the emission lines that would be produced if the gas were heated to high temperatures (see Figure 2.12).

The analysis of the ways in which matter emits and absorbs radiation is called spectroscopy. The observed relationships between the three types of spectra—continuous, emission line, and absorption line—are illustrated in Figure 2.16 and may be summarized as follows:
  1. A luminous solid or liquid, or a sufficiently dense gas, emits light of all wavelengths and so produces a continuous spectrum of radiation (Figure 2.11).
  2. A low-density hot gas emits light whose spectrum consists of a series of bright emission lines. These lines are characteristic of the chemical composition of the gas (Figure 2.12).
  3. A low-density cool gas absorbs certain wavelengths from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum. These lines are characteristic of the composition of the intervening gas. They occur at precisely the same wavelengths as the emission lines produced by the gas at higher temperatures (Figure 2.15). These rules are collectively known as Kirchhoffs laws, after the German physicist Gustav Kirchhoff, who published them in 1859.

Figure 2.16 Kirchhoffs Laws A source of continuous radiation, here represented by a lightbulb, is used to illustrate Kirchhoff’s laws of spectroscopy. (a) The unimpeded beam shows the familiar continuous spectrum of colors. (b) When the source is viewed through a cloud of hydrogen gas, a series of dark hydrogen absorption lines appears in the continuous spectrum. These lines are formed when the gas absorbs some of the bulb’s radiation and reemits it in random directions. Because most of the reemitted radiation does not go through the slit, the effect is to remove the absorbed radiation from the light that reaches the screen at left. (c) When the gas is viewed from the side, a fainter hydrogen emission spectrum is seen, consisting of reemitted radiation. The absorption lines in (b) and the emission lines in (c) have the same wavelengths.

Astronomical Applications

Once astronomers realized that spectral lines are indicators of chemical composition, they set about identifying the observed lines in the Sun’s spectrum. Almost all of the lines observed in the light received from extraterrestrial sources could be attributed to known elements (for example, many of the Fraunhofer lines in sunlight are associated with the element iron). However, some new lines also appeared in the solar spectrum. In 1868 astronomers realized that those lines must correspond to a previously unknown element. It was given the name helium, after the Greek word helios, meaning “Sun.” Only in 1895, almost three decades after its detection in sunlight, was helium discovered on Earth.

The development of spectroscopy is another example of the scientific method in action. (Discovery 1-1) As technology evolved and experimental measurement techniques improved, scientists realized that spectra were unique fingerprints of matter. They then went on to use this knowledge as a means of determining the composition of objects that were otherwise impossible to reach. Yet for all the information that nineteenth-century astronomers could extract from observations of stellar spectra, they still lacked a theory explaining how those spectra arose. Despite their sophisticated spectroscopic equipment, they knew scarcely any more about the physics of stars than did Galileo or Newton. The next step in the circle of scientific progress was an explanation of Kirchhoff’s empirical laws in terms of much more fundamental physical principles, just as Newtonian mechanics explained Kepler’s empirical laws of planetary motion. And just as with Newton’s laws, the new theory—now called quantum mechanics—opened the door to an explosion of scientific understanding far beyond the context in which it was originally conceived.

To understand how spectroscopy can be used to extract detailed information about astronomical objects from the light they emit, we must delve more deeply into the processes that produce line spectra.


What are absorption and emission lines? What do they tell us about the properties of the gas producing them?