In physics, taking up of matter or energy of one substance by another, such as a liquid by a solid (ink by blotting paper) or a gas by a liquid (ammonia by water). In physics, absorption is the phenomenon by which a substance retains the energy of radiation of particular wavelengths; for example, a piece of blue glass absorbs all visible light except the wavelengths in the blue part of the spectrum; it also refers to the partial loss of energy resulting from light and other electromagnetic waves passing through a medium. In nuclear physics, absorption is the capture by elements, such as boron, of neutrons produced by fission in a reactor.
When a liquid is absorbed by a substance such as blotting paper, the action depends on capillarity. Plants are enabled by their root fibres to absorb liquid matter into their tissues by osmosis. In the process of digestion, the intestines absorb compounds necessary for the nutrition of the body.
Absorption of gases by liquids The solubility of different gases in water varies considerably. One volume of water at 0°C/32°F and atmospheric pressure absorbs only 0.02 volumes of nitrogen, whilst ammonia at the same temperature and pressure dissolves to the extent of 1,050 volumes to 1 volume of water. The mass absorbed increases in proportion to the pressure (Henry's law), but decreases as the temperature increases, though not in exact proportion.
Absorption of gases by solids Some solids also have the property of absorbing gases, the best-known example being charcoal, which can absorb large quantities of ammonia, chlorine, phosgene, and other gases. For this reason, charcoal is an effective deodorant. After being heated in superheated steam charcoal becomes an even more efficient absorbent, and is said to be ‘activated’. Active charcoal is used in gas masks. Platinum black (finely divided platinum metal), if surrounded by a mixture of hydrogen and oxygen, absorbs so much of the gases, and therefore brings them into such intimate molecular contact, that sufficient heat is given out to ignite the rest of the gas.
Absorption of light and heat The energy of electromagnetic waves, for example light and heat radiation, is absorbed in discrete amounts, called quanta, and enough quanta may be absorbed by certain materials to allow photochemical reactions to take place. As well as absorption, some radiation may be transmitted with more or less disturbance of direction and some may be reflected. A particular substance may absorb rays of certain frequencies or wavelengths, allowing the others to be transmitted or reflected. Long waves, such as radio waves, can pass through optically opaque obstacles with very little absorption. Shorter heat waves are absorbed readily by dark substances such as lamp black, whilst the various wavelengths that correspond to the different colour sensations are variously affected by different substances, the result determining the colour as seen by transmitted light. Absorption and emission of electromagnetic radiation is the basis of spectroscopic methods employed to give information on the geometry and electronic structures of molecules. Molecules have distinct rotational, vibrational, and electronic energy levels, and can absorb electromagnetic radiation from a source. The absorption is quantized, that is, only an exact amount of energy will raise the molecule to its next energy level. It is possible to produce an absorption spectrum showing which wavelengths have been absorbed, and since different molecules and parts of molecules have different wavelength requirements, this serves as an analytical tool.
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