Named after the two scientists Johann Heinrich Lambert (1728-1777) and August Beer (1825-1863), this law states the correlation between the absorbance A, the path length traversed, and the concentration of the absorbent substance:
where the concentration c is stated in mol/l and the path length d in cm. The proportionality constant, the relative spectral absorption coefficient k, is a substance-specific function of the wavelength.
If monochromatic light is used in the appropriate concentration range, the Lambert-Beer Law is reliable with great accuracy. Consequently, the concentration of a substance dissolved in liquids or gases can be determined by measuring the absorbance A.