Absorption (electromagnetic radiation)

An overview of electromagnetic radiation absorption. This example discusses the general principle using visible light as specific example. A white beam source -- emitting light of multiple wavelengths -- is focused on a sample (the complementary color pairs are indicated by the yellow dotted lines). Upon striking the sample, photons that match the energy gap of the molecules present (green light in this example) is absorbed in order to excite the molecule. Other photons transmits unaffected and, if the radiation is in the visible region (400-700nm), the transmitted light appears as its complementary color. By comparing the attenuation of the transmitted light with the incident, an absorption spectra can be obtained.

In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is often called attenuation. Usually, the absorption of waves does not depend on their intensity (linear absorption), although in certain conditions (usually, in optics), the medium changes its transparency dependently on the intensity of waves going through, and the saturable absorption (or nonlinear absorption) occurs.

Quantifying absorption

Main article: Mathematical descriptions of opacity

There are a number of ways to quantify how quickly and effectively radiation is absorbed in a certain medium, for example:

• The absorption coefficient, and some closely related derived quantities:
  • The attenuation coefficient, which is sometimes but not always synonymous with the absorption coefficient
  • Molar absorptivity, also called "molar extinction coefficient", which is the absorption coefficient divided by molarity (see also Beer–Lambert law).
  • The mass attenuation coefficient, also called "mass extinction coefficient", which is the absorption coefficient divided by density (see also mass attenuation coefficient).
  • The absorption cross section and scattering cross-section are closely related to the absorption and attenuation coefficients, respectively.
  • "Extinction" in astronomy is equivalent to the attenuation coefficient.
• Penetration depth and skin effect,
• Propagation constant, attenuation constant, phase constant, and complex wavenumber,
• Complex refractive index and extinction coefficient,
• Complex dielectric constant,
• Electrical resistivity and conductivity.
• Absorbance (also called "optical density") and optical depth (also called "optical thickness") are two related measures of the total light-blocking power of a certain medium with a certain thickness.
• Percentage of the incoming light which gets absorbed.

All these quantities measure, at least to some extent, the same thing: How well a medium absorbs radiation. However, practitioners of different fields and techniques tend to conventionally use different quantities drawn from the list above. Fortunately, it is easy to convert from one measure to another, see Mathematical descriptions of opacity.

Measuring absorption

The absorbance of an object quantifies how much of the incident light is absorbed by it (not all photons get absorbed, some are reflected or refracted instead). This may be related to other properties of the object through the Beer-Lambert law.

Precise measurements of the absorbance at many wavelengths allow the identification of a substance via absorption spectroscopy, where a sample is illuminated from one side, and the intensity of the light that exits from the sample in every direction is measured. A few examples of absorption spectroscopy, in different parts of the spectrum, are ultraviolet-visible spectroscopy, infrared spectroscopy, and X-ray absorption spectroscopy.

Applications

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.

Understanding and measuring the absorption of electromagnetic radiation has an enormous variety of applications. Here are just a few examples:

• In meteorology and climatology, global and local temperatures depend in large part on the absorption of radiation by atmospheric gases (such as in the greenhouse effect) and the ground (see albedo).
• In medicine, X-rays are absorbed to different extents by different tissues (bone in particular), which is the basis for X-ray imaging. A concrete example is the Computation of radiowave attenuation in the atmosphere used in satellite link design.
• In chemistry and materials science, different materials and molecules will absorb radiation to different extents at different frequencies, which allows for material identification.
• In optics, sunglasses, colored filters, dyes, and other such materials are designed specifically with respect to which visible wavelengths they absorb and how much.
• In biology, photosynthetic organisms require that light of the appropriate wavelengths be absorbed within the active area of chloroplasts, so that the light energy can be converted into chemical energy within sugars and other molecules.

See also

• Hydroxyl ion absorption
• Water absorption
• Photosynthesis
• Ultraviolet-visible spectroscopy
• Solar cell
• Albedo
• Spectral line

References

• Thomas, Michael E. (January 2006). Optical Propagation in Linear Media: Atmospheric Gases and Particles, Solid-State Components, and Water. Oxford University Press, USA. pp. 3... (Chapter 1, 2, 7). ISBN 978-0-19-509161-8. http://books.google.com/books?id=EEodkQqPGs4C&printsec=frontcover&dq=Optical+propagation+in+linear+media+:+atmospheric+gases+and+particles,+solid+state+components,+and+water&cd=1#v=onepage&q=absorption&f=false. 
• ProfHoff, Ken Mellendorf, and Vince Calder (November 2010). "Reflection and Absorption". Physics Archive - Ask a scientist. Argonne National Laboratory. http://www.newton.dep.anl.gov/askasci/phy00/phy00232.htm. Retrieved 2010-11-14. 

External links