absorption line

spectroscopy

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July 23, 2026

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solar spectrum The visible solar spectrum (simulated), showing prominent Fraunhofer absorption lines.

An absorption line is a dark line in an absorption spectrum. This spectrum is created when a substance absorbs specific wavelengths of electromagnetic radiation, resulting in a pattern of dark lines or bands. This phenomenon is a key aspect of spectroscopy, a method used to analyze the properties of matter by observing how it interacts with electromagnetic radiation.12

In spectral methods, instruments measure the radiation absorbed, scattered, or emitted by a substance. Absorptiometry, a common technique, measures the amount of radiation absorbed by a sample. When a photon strikes a molecule, it can excite the molecule to a higher energy state, causing the radiation's intensity to decrease as it passes through. This absorption is dependent on the substance's chemical makeup and concentration. Stellar spectra, for example, reveal information about a star's temperature and composition through the absorption lines present.34

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Line spectrum
…permit an appreciable number of absorptions to occur. At the low surface temperatures of a red dwarf star, few electrons populate the second level of hydrogen, and thus the hydrogen lines are dim. By contrast, at very high temperatures—for instance, that of the surface of a blue giant star—the hydrogen…
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…permit an appreciable number of absorptions to occur. At the low surface temperatures of a red dwarf star, few electrons populate the second level of hydrogen, and thus the hydrogen lines are dim. By contrast, at very high temperatures—for instance, that of the surface of a blue giant star—the hydrogen…
Read More