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How do absorption and emission of electromagnetic radiation relate to the same frequencies in matter?

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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.

In physics, the relationship between the absorption and emission of electromagnetic radiation by matter is rooted in the discrete internal energy states of atoms and molecules.1

An atom or molecule can only change its internal energy by specific, discrete amounts. When a system transitions from a higher energy state to a lower one, it emits a quantum of radiation with a frequency $ν$ corresponding to the energy difference between those states ( $Δ E = h ν$ ). Conversely, a system can absorb a photon of that same frequency if the energy matches the difference required to promote the system from a lower state to a higher one.12

This principle of microscopic reversibility implies that any system capable of emitting radiation at a specific frequency is also capable of absorbing radiation at that same frequency. This relationship is fundamental to spectroscopy, where emission and absorption spectra serve as unique "fingerprints" for identifying the chemical composition of substances, from laboratory samples to distant stars. Furthermore, Kirchhoff’s radiation law notes that good emitters of thermal radiation are also good absorbers.3145