The Probert Encyclopaedia of Science & Technology

ABSORPTION

In chemistry absorption is the take up of a gas by a solid or liquid, or the take up of a liquid by a solid.
Absorption differs from adsorption in that the absorbed substance permeates the bulk of the absorbing substance. In physics the term absorption refers to the conversion of the energy of electromagnetic radiation, sound, streams of particles, etc., into other forms of energy on passing through a medium. A beam of light, for instance, passing through a medium, may lose intensity because of two effects: scattering of light out of the beam, and absorption of photons by atoms or molecules in the medium. When a photon is absorbed, there is a transition to an excited state. In biology the term absorption refers to the movement of fluid or a dissolved substance across a cell membrane. In animals, for example, soluble food material is absorbed into the circulatory system through cells lining the alimentary canal. In plants, water and mineral salts are absorbed from the soil by the roots.