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Beer–Lambert’s Law


Beer–Lambert’s Law

Beer–Lambert’s law (also called the Beer–Lambert–Bouguer law) explains how light is absorbed when it passes through a solution.

Statement

The absorbance of a solution is directly proportional to:

  • the concentration of the absorbing species, and
  • the path length of light through the solution.

Mathematical Form

A = ε · c · l

Where

  • A = absorbance (no units)
  • ε = molar absorptivity (L·mol⁻¹·cm⁻¹)
  • c = concentration of the solution (mol/L)
  • l = path length of the cell (cm)

Key Ideas (Easy to Remember)

  • Higher concentration → more absorption
  • Longer path length → more absorption
  • Used to find unknown concentrations using a spectrophotometer

Common Applications

  • Chemistry labs (concentration measurement)
  • Biochemistry (DNA/protein analysis)
  • Environmental testing (pollutants)
  • Medical diagnostics

Valid Only When

  • Light is monochromatic
  • Solution is dilute
  • No scattering or chemical interaction occurs

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