A law relating the reduction in luminous intensity of light passing through a material to the length of the light’s path through the material: i.e.

where ϵ‎ is the molar absorption coefficient, I is the intensity after passing through a sample of length l, I₀ is the incident intensity, and [J] is the molar concentration of species J. The Beer–Lambert law was formed empirically; however, it can be derived on the basis that the loss in intensity dI is proportional to the thickness dl of the sample, the concentration [J], and the intensity I (since the rate of absorption is proportional to the intensity). The Beer–Lambert law means that the intensity of light (or any other form of electromagnetic radiation) passing through a sample diminishes exponentially with the concentration and the thickness of the sample (for a given wave number).

Although the Beer–Lambert law is named after the work undertaken by Johann Heinrich Lambert (1728–77) in 1760 and August Beer (1825–63) in 1852, it was actually discovered by Pierre Bouguer (1698–1758) in 1729.