The Beer-Lambert Law Equation

Understanding A = εlc - what each variable means and how they relate.

Understanding the Beer-Lambert Law

A
Absorbance(dimensionless)

Also called optical density (OD). Measures the amount of light absorbed by a sample.

ε
Molar Absorptivity(L·mol⁻¹·cm⁻¹)

Also known as the extinction coefficient. A property of the substance at a given wavelength and temperature.

l
Path Length(cm)

Distance light travels through the sample. A standard cuvette has a path length of 1 cm.

c
Concentration(mol/L (M))

Molar concentration of the absorbing species in solution.

Relationship to Transmittance

Absorbance is directly related to transmittance (T), which is the fraction of incident light that passes through a sample. Transmittance is defined as the ratio of transmitted light intensity (I) to incident light intensity (I0).

A = −log10(T) = −log10(I / I0)

When T = 1 (100% transmission), A = 0. As the sample absorbs more light, T decreases and A increases.

Light Passing Through a Cuvette

I0Il

Incident light (I0) enters the cuvette. As it passes through the sample, some light is absorbed. The transmitted light (I) that exits is weaker, shown by the thinner arrow.

Continue Learning

Calculator

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Derivation

See how the Beer-Lambert law is derived from first principles using light intensity and transmittance.

Molar Absorptivity Table

Look up molar absorptivity values for common compounds at standard wavelengths.