Spectral lines are gaps in light’s frequency distribution caused by absorption or emission by gas. Absorption lines occur when light passes through gas, while emission lines are re-emitted light. Spectral lines provide information about composition and speed of astronomical objects.
Spectral lines are gaps in the ordinarily continuous frequency distribution in light. They result when the light emitted is partially absorbed by matter, usually a gas. One type of spectral line is an absorption line, which occurs when certain frequencies of light are absorbed by the emitted radiation. An emission line, on the other hand, is visible in light that has been absorbed and then re-emitted. These lines can provide astronomers with clues about the composition and speed of astronomical objects.
The first of the two types of spectral lines are absorption lines. Absorption lines occur when light is emitted from an object, such as a star, and then passes through a gas before reaching an observer. In this case, the light from a hot star will be strongly emitted over a wide range of frequencies; in other words, it will have a continuous radiation spectrum. However, if it passes through a cold gas, light with certain frequencies can be absorbed by the gas. When an observer analyzes the light, which originally came from the star, he will show absorption lines, obvious gaps in certain frequencies of radiation.
The second type of spectral line is an emission line. Starlight cannot be continuously absorbed by a gas because light contains energy. Rather, it is absorbed by the gas and then re-emitted at a later time. When re-emitted, the light contains only the frequencies that have been absorbed by the gas. Therefore, the frequencies of the emission lines should match the frequencies of the corresponding absorption lines. In fact, this has been verified by astronomers by observing the re-emitted light; the two types of straight lines are essentially inverses of each other.
Spectral lines exist because the light emitted by elements and chemicals has a continuous frequency spectrum. Rather than being a homogeneous entity, visible light is actually electromagnetic radiation within a certain range of frequencies. Even radiation invisible to the naked eye has a continuous frequency distribution, although it is concentrated outside the visible range. A scientific instrument known as a spectrometer can be used to analyze the frequencies of electromagnetic radiation.
Studying these lines can provide astronomers with two important pieces of information. First, by knowing which gases produce which absorption lines, spectral lines can help identify the composition of planetary atmospheres. Astronomers have used this technique to characterize the atmospheres of planets outside the solar system. Second, recognizable patterns of spectral lines can indicate the speed of astronomical objects: An object’s speed shifts the frequency of its radiation, including spectral lines.
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