## Wikipedia - Beer–Lambert law (en) Wikipedia - Fotoquímica (pt) Wikipedia - Legge di Lambert-Beer (it) Wikipedia - Transmittance (en) Wikipedia - قانون بير-لامبرت (ar) Wikipedia - ランベルト・ベールの法則 (ja) Beer–Lambert law (Beer–Lambert–Bouguer law)

https://doi.org/10.1351/goldbook.B00626
The @A00028@ of a beam of collimated monochromatic radiation in a homogeneous @I03353@ medium is proportional to the absorption path length, $$l$$, and to the concentration, $$c$$, or — in the gas phase — to the pressure of the absorbing species. The law can be expressed as: $A = \text{log}_{10}\left ( \frac{P_{\lambda }^{0}}{P_{\lambda }} \right ) = \varepsilon \:c\:l$ or $P_{\lambda }=P_{\lambda }^{0}10^{-\varepsilon \: c\: l}$ where the proportionality constant, $$\varepsilon$$, is called the molar (decadic) @A00037@. For $$l$$ in $$\text{cm}$$ and $$c$$ in $$\text{mol dm}^{-3}$$ or $$\text{M}$$, $$\varepsilon$$ will result in $$\text{dm}^{3}\ \text{mol}^{-1}\ \text{cm}^{-1}$$ or $$\text{M cm}^{-1}$$, which is a commonly used unit. The SI unit of $$\varepsilon$$ is $$\text{m}^{2}\ \text{mol}^{-1}$$. Note that @S05828@ must be used because the Beer–@L03445@ holds only if the spectral bandwidth of the light is narrow compared to spectral linewidths in the spectrum.
See:
absorbance
,
extinction coefficient
,
Lambert law
Source:
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2230 [Terms] [Paper]