The Beer –Lambert Law

When a monochromatic light of initial intensity Io passes through a solution in a transparent vessel, some of the light is absorbed so that the intensity of the transmitted light I is less than Io .There is some loss of light intensity from scattering by particles in the solution and reflection at the interfaces, but mainly from absorption by the solution.

Therelationship between I and Io depends on the path length of theabsorbing medium, l, and the concentration of the absorbing solution,c. These factors are related in the laws of Lambert and Beer

Lambert’s law

When a ray of monochromatic light passes throughan absorbing medium its intensity decreases exponentially as the length of the absorbing medium increases.

Beer’s law :

When a monochromatic light passes through an absorbing medium its intensity decreases exponentially as the concentration of the absorbing medium increases.

Transmittance:

The ratio of intensities is known as the transmittance (T) and this is usually expressed as percentage

Extinction

If logarithms are taken of the equation instead of a ratio then

The expression log10 Io/I is known as the extinction (E) or absorbance(A). The extinction is some times referred as optical density.

Therefore

A (or) E = k cl

where k is molar extinction co-efficient for the absorbing material atwave length l, c = molar concentration of the absorbing solution,l = path length in the absorbing material in cm.

If the Beer- Lambert law is obeyed correctly and l is kept constant, then a plot of extinction against concentration gives a straight line passing through the origin

Extinction, E=log10 Io/I

Extinction

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