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Why do atoms absorb only certain wavelengths of light?

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Atoms absorb only certain 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 change its internal energy by specific, allowed amounts corresponding to the difference between these states.123

When an atom interacts with electromagnetic radiation, it can only absorb a photon if the photon's energy exactly matches the energy gap between its current state and a higher-energy state. This relationship is defined by the equation $E = h ν$ , where $E$ is the energy difference, $h$ is Planck’s constant, and $ν$ is the frequency of the light. Because the frequency is directly related to the wavelength ( $λ = c / ν$ ), only specific wavelengths of light can be absorbed to trigger these transitions.1324

Each chemical element has a unique set of energy levels, meaning these absorbed wavelengths act as a distinct "fingerprint" used in spectroscopy to identify the composition of substances.32