Beer–Lambert law

The Beer–Lambert law states that a light-absorbing substance’s absorbance is proportional to its concentration and the distance light travels through it.

Absorbance: A logarithmic measure of how much light a sample absorbs, defined from the ratio of incident to transmitted intensity. It is the measured quantity the law relates linearly to concentration and path length.
Ultraviolet–visible spectroscopy: A technique that measures how molecules absorb ultraviolet and visible light as a function of wavelength. It commonly uses absorbance and the law to estimate solution concentrations.
Electromagnetic radiation: Energy propagated through space as coupled oscillating electric and magnetic fields. The law describes how matter attenuates this radiation as it passes through a sample.
Stray light: Unwanted light reaching a detector without following the intended optical path or wavelength selection. It can make measured absorbance too low, especially for strongly absorbing samples.
Pierre Bouguer: An eighteenth-century French mathematician and physicist who studied the attenuation of light in transparent materials. His work established an early exponential description of light loss through a medium.
Molar attenuation coefficient: A wavelength-dependent constant describing how strongly a chemical species attenuates light per unit concentration and path length. It is the proportionality factor that makes different substances absorb light to different degrees.
Calibration curve: A plot relating an instrument’s response to known amounts or concentrations of an analyte. Standards establish the absorbance–concentration relation used to quantify unknown samples.
Molecular absorption: The transfer of photon energy to a molecule through transitions between quantized energy states. Absorption by molecular transitions produces the attenuation measured by the law.
Chemical deviation from Beer–Lambert law: A departure from linear absorbance–concentration behavior caused by concentration-dependent chemical speciation or interactions. Association, dissociation, or reaction can change the absorbing species as concentration varies.
Johann Heinrich Lambert: An eighteenth-century Swiss polymath who formulated a law relating light intensity to the distance traveled through an absorbing medium. His contribution accounts for the path-length dependence in the named law.

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