Absorption, the process by which matter or radiant energy is taken up internally, or assimilated, by a substance. Matter that is absorbed can be gaseous, liquid, or solid. Radiant energy includes light, heat, X rays, ultraviolet rays, radio waves, and sound.

Adsorption, a related phenomenon, is sometimes confused with absorption. Adsorption is the process by which a substance attracts and holds particles of another substance on its surface. Absorption always involves internal penetration of a substance, whereas adsorption involves surface retention only.

Absorption of Matter

Many examples of the absorption of matter occur in everyday experience. A readily observed example is provided by a sponge. It is a porous solid that will absorb water and other liquids.

In animals and plants, the absorption of matter is of vital importance. Digested food in the small intestine is absorbed by the bloodstream and distributed to all parts of the body. Plant roots absorb water and nutrients from soil, and leaves absorb oxygen and carbon dioxide from air.

The production of energy in nuclear reactors depends on the fact that subatomic particles called neutrons are absorbed by the nuclei of uranium or plutonium atoms. The absorbed neutrons cause the nuclei to fission, or split, and in the process of splitting they release energy together with additional neutrons.

Absorption of Radiant Energy

Radiant energy that strikes a substance can be absorbed, transmitted, or reflected. The radiant energy that is absorbed by a substance is converted into heat or some other form of energy. The amount of energy absorbed depends on the physical nature of the substance and the character of the radiant energy. A mirror reflects most of the light that shines on it, while most of the light that strikes an untinted glass window is transmitted through the window. Charcoal and soot, on the other hand, absorb virtually all light that strikes them.

The earth's atmosphere absorbs part of the energy radiated by the sun and thus prevents this energy from reaching the earth's surface. If this atmospheric absorption did not occur, life probably could not exist on earth. This is because the atmosphere absorbs ultraviolet rays, some of which are known to be lethal to both animals and plants. Light from distant stars is absorbed both by the atmosphere and by interstellar gas and dust.

The color of an object that is not self-luminous is determined by the wavelengths of light that the object absorbs, transmits, and reflects.

All materials used in building construction absorb sound waves to a certain extent; those that are designed to absorb a large percentage of sound waves are known as acoustical materials. Such materials are usually lightweight, porous, flexible, or compressible. Gypsum plaster and perforated cellulose-fiber tile are among the common acoustical materials.