Beer Lamberts Law:

In optics, the Beer-Lambert law, also known as Beer's law or the Lambert-Beer law or the Beer-Lambert-Bouguer law (in fact, most of the permutations of these three names appear somewhere in literature) relates the absorption of light to the properties of the material through which the light is traveling.

The law states that there is a logarithmic dependence between the transmission (or transmissivity), T, of light through a substance and the product of the absorption coefficient of the substance, a, and the distance the light travels through the material (i.e. the path length), l. The absorption coefficient can, in turn, be written as a product of either a molar absorptivity of the absorber, e, and the concentration c of absorbing species in the material, or an absorption cross section, s, and the (number) density N of absorbers.

There are at least five conditions that need to be fulfilled in order for Beer's law to be valid. These are: 

• The absorbers must act independently of each other;  
• The absorbing medium must be homogeneously distributed in the interaction volume and must not scatter the radiation; 
• The incident radiation must consist of parallel rays, each traversing the same length in the absorbing medium;  
• The incident radiation should preferably be monochromatic, or have at least a width that is more narrow than the absorbing transition; and 
• The incident flux must not influence the atoms or molecules; it should only act as a non-invasive probe of the species under study. In particular, this implies that the light should not cause optical saturation or optical pumping, since such effects will deplete the lower level and possibly give rise to stimulated emission.

If any of these conditions is not fulfilled, there will be deviations from Beer's law.

Beer's law can be applied to the analysis of a mixture by spectrophotometry, without the need for extensive pre-processing of the sample. An example is the determination of bilirubin in blood plasma samples. The spectrum of pure bilirubin is known, so the molar absorbance is known. Measurements are made at one wavelength that is nearly unique for bilirubin and at a second wavelength in order to correct for possible interferences.

The law is used widely in infra-red spectroscopy for analysis of polymer degradation and oxidation. The carbonyl group absorption at about 6 microns can be detected quite easily, and degree of oxidation of the polymer calculated.