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:
$A = \kappa \cdot c \cdot L$
where the concentration c is stated in mol/l and the path length L in cm. The proportionality constant, the relative spectral absorption coefficient κ, is a substance-specific function of the wavelength. Monochromaticity
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.