The capture of photons by atoms, molecules,
or ions, which results in a decrease in the intensity of light, or other
electromagnetic radiation, as it passes
through a substance. Absorption of electromagnetic radiation is the
combined result of Compton scattering (see Compton
effect), σ, and photoelectric absorption, τ.
If the intensity of the incident beam of radiation is I0,
the intensity of the beam attenuated by absorption, If
, may be written as:
If = I0 e-μρt
where t is the thickness of the absorbing medium and μ
= σ + τ.
In astronomy, absorption occurs, for example, in the atmospheres of
stars, planets, and moons, and in the interstellar
medium. Absorption in Earth's atmosphere is one of the causes of
atmospheric extinction; between
the stars it occurs as interstellar
Absorption at a specific wavelength results in absorption
lines or absorption bands; otherwise
it is known as continuous absorption. Compare with
- More generally, absorption is any process by which a substance incorporates
another substance into itself, or takes in radiant or sound energy.
In chemistry it is distinguished from adsorption
in which the adsorbed substance merely adheres to the surface of the
absorbent. In absorption the absorbed matter permeates all of the absorber;
this includes a gas taken in by a liquid, such as carbon dioxide by
sodium hydroxide, and a liquid or gas absorbed by a solid, such as water
by a gel. In air-pollution control and the
chemical industries gas absorption is a key process, while the varying
extent to which different surfaces absorb sound
is an important factor in acoustic design.
- In nuclear physics, neutrons produced
by fission in a nuclear reactor
are absorbed by materials such as boron
in control rods or other materials
making up the radiation shielding around the reactor.
- In physiology, absorption refers to the uptake of fluids or other
substances by the tissues of the body. Digested food is absorbed into
the blood and lymph
from the gastrointestinal tract. Most absorption of food occurs in the
small intestine – in the
jejunum and ileum – although alcohol is readily absorbed from
the stomach. The small intestine is lined with minute finger-like processes
(villi), which greatly increase its surface
area and therefore the speed at which absorption can take place.
AND OPTICAL PHENOMENA