August Beer (; 31 July 1825 – 18 November 1863) was a German physicist, chemist, and mathematician of Jewish descent. Beer published a paper in the field of spectroscopy on the absorption of red light in colored aqueous solutions of various salts, Beer makes use of the fact, derived from Bouguer's and Lambert's absorption laws, that the intensity of light transmitted through a solution at a given wavelength decreases exponentially with the path length d and the concentration c of the solute (the solvent is considered non-absorbing). The “Absorption Coëfficient” that Beer defined is actually the transmittance (or transmission ratio), T = I / I<sub>0</sub>. In Beer's formulation: "the transmittance of a concentrated solution can be derived from a measurement of the transmittance of a dilute solution."
The transmittance measured for any concentration and path length can be normalized to the corresponding transmittance for a standard concentration and path length. Beer conducted a number of experiments to confirm this empirical law, and to define a standard concentration of 10%, and a standard path length of 10 cm.
Beer's law
Beer's law, also called the Beer–Lambert law, in spectroscopy, is the physical law stating that the quantity of light absorbed by a substance dissolved in a non-absorbing solvent is directly proportional to the concentration of the substance and the path length of the light through the solution. Beer's law is commonly written in the form
<math display="block">A = \varepsilon cl,</math>
where A is the absorbance, c is the concentration in moles per liter, l is the path length in centimeters, and ε is a constant of proportionality known as the molar extinction coefficient. The law is accurate only for dilute solutions; deviations from the law occur in concentrated solutions because of interactions between molecules of the solute, the substance dissolved in the solvent.
Gallery
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File:Photometer devised by Beer.jpg|Photometer devised by Beer
File:Beer's Law Plot.jpg|Example plot displaying the Beer–Lambert Law
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