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A. R. Forouhi and I. Bloomer deduced dispersion equations for the refractive index, n, and extinction coefficient, k, which were published in 1986 [1] and 1988. [2] The 1986 publication relates to amorphous materials, while the 1988 publication relates to crystalline.
The SI unit of molar absorption coefficient is the square metre per mole (m 2 /mol), but in practice, quantities are usually expressed in terms of M −1 ⋅cm −1 or L⋅mol −1 ⋅cm −1 (the latter two units are both equal to 0.1 m 2 /mol).
Extinction coefficient refers to several different measures of the absorption of light in a medium: Attenuation coefficient , sometimes called "extinction coefficient" in meteorology or climatology Mass extinction coefficient , how strongly a substance absorbs light at a given wavelength, per mass density
[1]: 128 It is related to the absorption coefficient, , through: [39]: 41 = These values depend upon the frequency of the light used in the measurement. That κ corresponds to absorption can be seen by inserting this refractive index into the expression for electric field of a plane electromagnetic wave traveling in the x -direction.
Most commonly, the quantity measures the exponential decay of intensity, that is, the value of downward e-folding distance of the original intensity as the energy of the intensity passes through a unit (e.g. one meter) thickness of material, so that an attenuation coefficient of 1 m −1 means that after passing through 1 metre, the radiation ...
There are also weaker dependencies on temperature, pressure/stress, etc., as well on precise material compositions (presence of dopants, etc.); for many materials and typical conditions, however, these variations are at the percent level or less. Thus, it's especially important to cite the source for an index measurement if precision is required.
The SI unit of mass attenuation coefficient is the square metre per kilogram (m 2 /kg). Other common units include cm 2 /g (the most common unit for X-ray mass attenuation coefficients) and L⋅g −1 ⋅cm −1 (sometimes used in solution chemistry). Mass extinction coefficient is an old term for this quantity. [1]
The extinction efficiency in this approximation is given by Q = 2 − 4 p sin p + 4 p 2 ( 1 − cos p ) , {\displaystyle Q=2-{\frac {4}{p}}\sin p+{\frac {4}{p^{2}}}(1-\cos p),} where Q is the efficiency factor of scattering, which is defined as the ratio of the scattering cross-section and geometrical cross-section π a 2 .