The Columbia Encyclopedia, Sixth Edition. 2001-07.
Beer’s law
[for August Beer], physical law stating that the quantity of light absorbed by a substance dissolved in a nonabsorbing solvent is directly proportional to the concentration of the substance and the path length of the light through the solution; the law is sometimes also referred to as the Beer-Lambert law or the Bouguer-Beer law. Beer’s law is commonly written in the form A=cl, where A is the absorbance, c is the concentration in moles per liter, l is the path length in centimeters, and 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.
cl, where A is the absorbance, c is the concentration in moles per liter, l is the path length in centimeters, and 
