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(Or Beer's law, Bouguer–Lambert law; sometimes called Lambert's law of absorption.) Attenuation of a beam of light by an optically homogeneous (transparent) medium.
First stated by Pierre Bouguer in his Essay on the Gradation of Light (1729), Bouguer also recognized that this law is independent of the attenuation mechanism. Although exponential attenuation with distance is attributed to Bouguer, Lambert, or Beer, the historically correct term is Bouguer's law, which is also valid for turbid media to the extent that multiple scattering is negligible. By a slight distortion of history we might say that Beer's law is an extension of Bouguer's law to solutions of fixed thickness but variable concentration of the absorbing solute. Mathematically, the law is written:
where I0 is the incident radiance, I is the transmitted radiance, and τ is the pathlength.
Middleton, W. E. K. 1961. Pierre Bouguer's Optical Treatise on the Gradation of Light. Translation.
Pfeiffer, H. G., and H. A. Liebhafsky 1951. J. Chem. Educ.. 28. 123–125.
Malinin, D. R., and J. H. Yoe 1961. J. Chem. Educ.. 38. 129–131.