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Why do atoms emit or absorb specific wavelengths of light?

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Bohr model diagram of a hydrogen atom showing a green nucleus labeled H, a single electron orbit, and text with atomic mass 1.008 and electron configuration 1. — atom The Bohr model describes the structure of atoms, in this case hydrogen.

Atoms emit or absorb specific wavelengths of light because they possess discrete internal energy levels, known as quantum states. According to the principles of quantum mechanics, an atom can only exist in these specific, allowed energy states.123

When an atom transitions between these states, it must gain or lose an exact amount of energy equal to the difference between the two levels.12
* Emission: When an electron moves from a higher-energy state to a lower-energy state, the atom releases the excess energy as a photon of [electromagnetic radiation].14
* Absorption: When an atom absorbs a photon, it uses that energy to transition from a lower-energy state to a higher-energy state.42

The energy of the photon (E) is directly related to its frequency ( $ν$ ) and wavelength ( $λ$ ) by the equation $E = h ν$ , where h is Planck’s constant. Because each chemical element has a unique set of allowed energy levels, the specific wavelengths of light emitted or absorbed act as a "fingerprint" that can be used to identify the atom's composition.12