In chemistry, absorption (also called gas absorption, gas scrubbing, or gas washing) is the transfer of one or more species from the gas phase to a liquid solvent. The species transferred to the liquid phase are called solutes or absorbates. Absorption does not change the chemical species present in the system. Absorption is used to separate gas mixtures, remove contaminants, or recover valuable chemicals. The process of removing the absorbed solute from the solvent is called stripping. Absorbers are usually used with strippers to allow regeneration (or recovery) and reuse of the absorbent. Because stripping is not perfect, the absorbent recycled to the absorber will contain species present in the vapor entering the absorber. When water is used as an absorbent, it is usually separated from the solute by distillation rather than stripping.

In physics, absorption of electromagnetic radiation is the process by which matter (typically electrons bound in atoms) absorbs the energy of a photon – thereby converting electromagnetic energy to internal energy of the absorber (e.g. thermal energy). Absorption of electromagnetic radiation is the combined result of the Compton effect) and photoelectric absorption. A notable effect (attenuation) is the gradual reduction in intensity of light waves as they propagate through a medium. Although the absorption of waves usually does not depend on their intensity (linear absorption), under certain conditions (optics) the transparency of the medium changes by a factor that varies as a function of wave intensity, and saturable (or nonlinear) absorption occurs.

In acoustics, absorption refers to the process by which a material, structure, or object absorbs sound energy when sound waves strike it, as opposed to reflecting the energy. Some of the absorbed energy is converted to heat and some is transmitted through the absorbing body. The energy converted to heat is said to be “lost”.

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