Lambert Beer´s Law

Lambert Beer´s Law

The Lambert Beer´s law (also known as Lamber-Beer, Beer–Lambert law or Beer’s law) describes the weakening of light as it passes through an absorbing medium. It shows how absorbance depends on concentration and path lenght.

Definition

The Lambert-Beer law can be expressed as:

A = ε · c · l

where:

A = absorbance

ε = molar absorptivity

c = concentration

l = path length

Absorbance can also be described as:

A = log10(I0 / I)

where:

I0 = incident light intensity

I = transmitted light intensity

Physical Background

When light passes through a solution, a part of the light is absorbed by molecules in the sample. The higher the concentration, the more light is absorbed. Aborbance also increases with a longer path length.

The law combines two proportional relationships:

A ∝ c

and

A ∝ l

Together, these give:

A ∝ c · l

By adding the proportionality constant ε, the final equation becomes:

A = ε · c · l

Applications

The Lambert-Beer law is used in:

spectrophotometry

protein and nucleic acid quantification

environmental analysis

clinical laboratory diagnostics

determination of unknown concentrations using calibration curves

Limitations

The law may vary under non-ideal conditions, such as:

very high concentrations

non-monochromatic light

scattering or turbidity

chemical changes in the sample

Summary

The Lambert-Beer Law is a basic idea in chemistry and biochemistry. It shows how absorbance depends on concentration and path length, which is important for quantitative spectroscopic analysis.

References

LibreTexts Chemistry: The Beer–Lambert Law

DocCheck Flexikon: Lambert-Beer’sches Gesetz

Nelson and Cox: Lehninger Principles of Biochemistry

Spectroscopy: Understanding the Limits of the Bouguer-Beer-Lambert Law