In physics, absorption of electromagnetic radiation is the way in which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed into internal energy of the absorber, for example thermal energy. The reduction in intensity of a light wave propagating through a medium by absorption of a part of its photons 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 saturable absorption (or nonlinear absorption) occurs.
(詳細は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
All these quantities measure, at least to some extent, how well a medium absorbs radiation. However, practitioners of different fields and techniques tend to conventionally use different quantities drawn from the list above.
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