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How did Gustav Kirchhoff show that pure substances have characteristic spectra?

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Gustav Kirchhoff demonstrated that pure substances have characteristic spectra by overcoming the limitations of earlier research, which had been hindered by the presence of impurities in test samples. By utilizing carefully purified substances, Kirchhoff and his colleague Robert Bunsen were able to show that every element emits a unique pattern of individual wavelengths when heated to incandescence.12

When this light is separated by a prism, it produces a distinct set of bright lines—an emission spectrum—that serves as a specific signature for that element. This discovery allowed Kirchhoff to establish the theory of spectrum analysis, a powerful tool for identifying chemical elements. Furthermore, Kirchhoff observed that gases absorb the same wavelengths they emit when heated, a principle he used to explain the dark Fraunhofer lines in the Sun’s spectrum, thereby enabling the determination of the Sun's chemical composition.21