BeerLambert

The Beer–Lambert law, often called Beer-Lambert-Bouguer law, describes how the intensity of light decreases as it passes through a sample due to absorption by its solute. In its common form, A = εlc, where A is the absorbance (a logarithmic measure of light loss), ε is the molar absorptivity (extinction coefficient) with units of L/(mol·cm), l is the path length of the light through the sample in centimeters, and c is the solute concentration in moles per liter. Since transmittance T = I/I0, absorbance is related by A = -log10(T).

The law applies to monochromatic light and to solutions where only a single absorbing species contributes significantly to the signal. It underpins quantitative spectrophotometry: by measuring absorbance at a chosen wavelength and using a calibration curve, one can infer concentration. In practice, a typical cuvette path length is 1 cm, and ε is determined for the solute and wavelength of interest.

Limitations and deviations arise at higher concentrations, where interpolation breaks down, scattering or fluorescence can affect measurements, and ε can depend on wavelength, temperature, and solvent. The linear relationship holds best for dilute solutions and optically homogeneous samples. Nonlinearities may require corrections or alternative methods.

Historically, the relationship was developed by Beer in the 1850s and Bouguer extended it; the combined form is sometimes called the Beer-Lambert-Bouguer law.