Absorption (electromagnetic radiation)

The term absorption ( absorptio Latin " absorption ", sometimes also " absorption " ) generally refers to the absorption or in- itself - recording of something and is not to be confused with the adsorption. In physics, it may be the absorption of a wave (electromagnetic waves, sound waves), a single particle (such as a free electron in an atom ), or a particle beam (particle beam ) in a substance or body.

On some areas of work, the term absorption accordingly has no entirely fixed definition, but is used somewhat differently depending on the effect under consideration. Examples thereof are mentioned below in the X-ray and gamma radiation and neutrons.

In the transmission of a wave or radiation through a substance or body, the absorption leads to a weakening of the original wave. Other effects that can cause a similar attenuation as the scattering or reflection are then combined in the optical system with the absorption, the term optical density ( absorbance also ).

Absorption of waves and particle

In the absorption of waves in an absorbent, homogenous material, the probability of absorption per unit path at low energies at each depth is equal. Then an exponential law, the Bouguer - Lambert's law - often just called Lambert 's law (not to be confused with the Lambert's cosine law ). I0 is the initial current, is the d remaining after passing through the layer thickness current I ( d):

( Derivation of the law: see absorption law ). Here, the μ on the properties of the absorbent material and often also by the energy ( quantum energy and particle velocity ) of the radiation -dependent absorption coefficient. Its reciprocal is the penetration depth. For him, the thickness of the half-value layer can be calculated.

Often, however, experience side effects, which lead to very different rules, as can be seen in the picture. There are different causes:

  • Formation of secondary electrons that are thrown in the irradiated material.
  • At high speeds protons ionize only slightly.
  • Electrons have a sharply defined maximum depth of penetration because of their electrical charge. Rule of thumb: in body tissue (meat ) per 2 MeV 1 cm.
  • High energetic photons and electrons in common is that they do not give their maximum dose deep to the skin surface, but a few millimeters.

The relatively thick air layer of the earth acts together with its magnetic field as a very effective absorber or particle deflector towards the magnetic poles of the earth for high-energy particles from the sun or from space. Depending on the particle type and energy, therefore, the radiation activity in the Van Allen belts increases sharply, the surface is very well protected. In the vicinity of the magnetic poles, these particles produce aurora; see also air showers.


The absorption of sound takes place by converting the power of the sound ( air-borne sound, impact sound, and seismic waves ) to thermal energy in a damping medium, or in the boundary layers - for example, between the air in the spreads of the sound, and a solid surface - takes place. It is, inter alia, frequency and temperature dependent. In air, the sound absorption is due to different thermo- dynamic processes, while it is considerably higher than that in solids in gases.

Electromagnetic waves

If electromagnetic radiation can be absorbed in a material, the strength of the absorption material is described by a parameter, the degree of absorption, which is usually of a plurality of parameters (temperature, wavelength) is dependent.

Visible light

The light absorption to surfaces, or when passing through matter is material and frequency dependent in general. The amount of absorbed or scattered photons depends on the way in addition to the frequency-dependent extinction coefficient and thickness of the layer of the material ( cf. Lambert -Beer 's law).

As a function of the possible band structure of the molecules different frequency ranges of light absorbed can be different, i.e. depending on the color, the absorption varies greatly (see the resonance absorption, the Fraunhofer lines). Adjacent frequency components are depending on the material and angle of incidence of the light either reflected or transmitted. For example, a yellow- appearing surface is irradiated with white light, the green and red light is reflected / transmitted and absorbs blue light (see, color synthesis ). In the absorption of light, the absorbed energy is not only converted into heat, but can be lost through other mechanisms such as fluorescence and scattering by aerosols.

As already mentioned, the absorbent (strong part ) is dependent on frequency. The cause lies in the band structure of the material in which photons of certain energy atoms or molecules stimulate the quantum transitions with exactly this energy difference in the electron shell or in their molecular vibrations (mostly in infra-red light ) hold. Interfacial cause additional, dependent on the refractive index difference, the direction of incidence and the polarization reflection. In diffuse reflection one speaks in this context of remission.

The light passing through a plate including absorption can be derived directly from the complex refractive index of the Kramers -Kronig relations. Thus, the electromagnetic interaction is directly related to a material property in relationship.

X-ray and gamma radiation

Also during passage of X-rays and gamma rays by matter, the probability of absorption proportional to the thickness d of the irradiated material, as well as the probability of scattering. This results in an exponential decrease in intensity with increasing thickness:

Here, the absorption coefficient, measured in m- 1, the number of atoms in the material per cubic meter, and σ the cross section for absorption. In optics, this law is called the Lambert- Beer law. One can describe the attenuation of the beam by a half- thickness. It is inversely proportional to the absorption coefficient.

Processes with energy loss

Often of absorbance ( only ) counted those processes in which a photon energy discharges its partially or completely. In the energy of the gamma radiation are:

  • The photoelectric effect, in which an electron having the energy of the photon ( minus the ionization energy of the atom concerned ) is released,
  • Compton effect the photon is its energy to an electron in part from, and the scattered photon has a reduced power,
  • At energies of at least 1.022 MeV pairing in the vicinity of the nucleus: in place of the photon occur a positron and an electron.

The cross section of each of these processes depends on the energy of the photon and the atomic number of the material. The photoelectric effect is predominant for low energies and high atomic number, the pair production for high energy and high atomic number, the Compton effect for medium energies and low atomic number.

The total cross-section for absorbing the sum of the individual cross sections of the various processes of the so- defined absorption is:

The electrons released from all three processes can continue to operate ionizing turn with sufficient energy.

Attenuation of the incident beam

To "absorb " but sometimes every process is counted, the distance a photon from the incident beam, with or without energy conversion. Then in gamma and X- radiation, the Rayleigh scattering must be taken into account, the only changes the direction of flight of the photon. The total cross section is then

The thus defined absorption coefficient, the linear attenuation coefficient is to be used in the calculation of the exponential decrease described above. Even then, this only applies to certain idealizations, such as a thin, linear beam. In Radiographic about a thick, solid wall it does not apply because it eg comes to Into scattering here in the beam.

Remote Sensing

In remote sensing, the term absorption of the receiving energy of electromagnetic radiation through the atmosphere and the earth surface. Thus energy is temporarily stored and re-emitted according to Planck's radiation law in any direction. Then re-emitted by the sun heated surface radiation in the wavelength range of the mid-infrared ( approximately 8-14 microns ). This radiation is absorbed by clouds and greenhouse gases, and thus delay into space and back to Earth re-emits ( greenhouse effect). Therefore, it is clear nights colder than covered.

LIDAR is able to provide a layer profile of the concentration of trace gases. Here one works with specific wavelengths that selectively stimulate the molecules of trace gases and are absorbed and re-emitted. Also, a profile of the wind speed can be obtained ( Doppler shift of backscattered radiation).

Color or wavelength-dependent absorption of the surface helps to distinguish between different coverages. One uses the visible and infrared spectral range, to determine kinds of vegetation and temperatures.

With satellite-based radar can win surface profiles, but also determine wave frequency and height.

Radio waves

Radio waves for communication or the radar be in the atmosphere absorbed by free carriers ( ionization ) as well as rain and snow or hail, reflected and scattered.

So spread medium waves during daytime bad ( ionization of the lower atmosphere by solar radiation), however, is well at night. Because the absorption in the plasma is polarization dependent, linkszirkuläre radio waves of long wavelength ( medium and short wave) are almost completely absorbed in the northern hemisphere. Only rechtszirkuläre waves are reflected at the bottom of the ionosphere when sufficient incident at a shallow angle; Short waves pass so the whole earth.

While the absorption of microwaves on precipitation in message transmission cause great problems with preparing ( radio, up-and down - links of satellite communication ), one is with precipitation radar ( ground- based) or weather radar on board of ships and aircraft capable of precipitation areas and even their to determine drops or hailstones and wind speed. Here, the Rayleigh scattering is significant - the lower the wavelength, the more scattering particles with dimensions substantially less than the wavelength. The wind speed is determined based on the Doppler shift of the backscattered waves.

When solar storms may grind to a halt to radio communications, if the atmosphere is ionized down to low layers and radio waves absorbed.

For the realization of radio measuring cell walls either high absorptance must own or be very large surface area, as shown in the picture. Then lower absorption coefficient of the material is sufficient. Because of the very troubled surface hardly specular reflections may occur.

Free neutrons

Also in connection with neutron absorption, the term is not used consistently. As absorption on the one hand from the nuclear physics point of view, each receiving a free neutron are called in an atomic nucleus, regardless of how the core behaves afterwards. A uranium core can be split, for example by absorption. This 2 to 3 neutrons are released; the absorption has then led to a neutron multiplication.

In reactor physics and physics of fusion reactor blankets, however, often involves the calculation of a " neutron balance ". Here, with the absorption (only ) those summarized processes that reduce the total number of free neutrons in the observed volume, such as (N, ) -, ( n, p) -, (n, ) reactions. Processes such as nuclear fission or (n, 2n ) reactions include here, however, to production, because they increase the number of neutrons.