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Absorption coefficient

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Absorption coefficient diagram
The absorption coefficient of a substance relates the optical density it provides with the sample length and concentration. It is essentially a cross-section of interaction between a molecule and a photon, and is sometimes expressed as such.
Cell length

Absorption coefficient
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While the SI unit is the absorption cross-section σ, the extinction coefficient ε (epsilon) is more often used. Typical ε values for visible wavelength absorption maxima are of the order of 1000 - 100,0000.
A related calc can be used to calculate concentration from known OD, path length and absorption coefficient.