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Spectroscopy Defined

Excerpt from Field Guide to Spectroscopy

Spectroscopy is the study of matter using electromagnetic radiation (light).

Spectroscopy is based on quantum mechanics, the prevailing theory of the behavior of atoms and molecules. One of the conclusions of quantum mechanics is that the energies of the various forms of motion within atoms and molecules are limited to certain discrete values; that is, they are quantized. When an atomic or molecular system absorbs or emits light, the system goes from one quantized energy level to another. The Bohr frequency condition states that the difference in the energy levels must equal the energy of the light absorbed or emitted. Spectroscopy uses this principle to probe the energy levels of the matter under study. Ultimately, spectroscopy helps us learn how matter and energy interact.

 Most of what we know about the universe comes from studying the interaction of light and matter, which makes spectroscopy a fundamental technique in understanding the world around us.

Light is not the only probe used in spectroscopy. Several types of spectroscopy use magnetic fields in conjunction with light to probe the nature of matter. In some cases, like nuclear magnetic resonance (NMR) spectroscopy, it is clear from the name of the method that magnetic fields are involved. In other cases, like Zeeman spectroscopy, it is not clear from the name of the technique.

Spectrometry is a more restrictive term. It refers to the measurement of the intensity of absorption or emission of light at one or more specific wavelengths, rather than a range of wavelengths.


D. W. Ball, Field Guide to Spectroscopy, SPIE Press, Bellingham, WA (2006).

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David W. Ball
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