The process by which a system takes in energy from incoming waves or particles.Absorption is the process by which a physical system takes in energy from incoming waves or particles. In physics, it is the fundamental counterpart to emission: while emission releases energy, absorption captures it, converting it into internal energy of the absorbing medium. This concept is central to understanding how light interacts with matter, how sound is damped, and how particles lose energy as they pass through materials.
The quantitative understanding of absorption began in the 18th century with the work of Johann Heinrich Lambert and August Beer, who formulated the law relating light absorption to the properties of the material through which it passes.
When a wave or particle encounters a medium, it can transfer its energy to the atoms or molecules of that medium. For electromagnetic radiation, absorption typically occurs when the energy of a photon exactly matches the energy difference between two quantum states of an atom or molecule, causing an electron to jump to a higher energy level. The absorbed energy is then either re-emitted, converted to heat through collisions, or stored in internal degrees of freedom. For particles, absorption can occur through inelastic scattering or nuclear reactions, where the particle's kinetic energy is transferred to the target nucleus.
Absorption is exploited in solar panels to convert sunlight into electricity, in spectroscopy to identify chemical substances by their absorption spectra, and in acoustic engineering to design soundproofing materials.
A perfectly black body, which absorbs all incoming radiation, would be invisible to the human eye — you could only see it by the light it emits due to its own temperature.
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