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How does spectroscopy use absorbed and emitted radiation to identify materials?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Spectroscopy identifies materials by measuring how atoms, ions, or molecules interact with electromagnetic radiation through absorption and emission processes. Because these particles possess unique, discrete quantum states, they can only absorb or emit energy corresponding to the specific differences between these energy levels.1234

Absorption Spectroscopy: When radiation passes through a sample, particles absorb specific wavelengths required to excite them from a lower to a higher energy state. These absorbed wavelengths are missing from the transmitted spectrum, creating dark lines that act as a unique fingerprint for the material.34
Emission Spectroscopy: Samples are excited—often via heat or electrical discharge—to higher energy levels. As these particles relax back to lower states, they release the excess energy as photons of characteristic frequencies. Measuring these bright spectral lines allows for the identification and quantification of the material's composition.143

By analyzing these spectral patterns, scientists can determine the chemical makeup and physical structure of substances ranging from terrestrial samples to distant stars.12