Beer's Law

No it's not a law about beer as in ale. Beer's law, or the Beer-Lambert law describes the relationship between absorbance of light by a substance, the distance the light travels through the substance, and the concentration of the substance.

A=elc

Where A* is absorbance; e is the molar absorptivity of the substance in L/mole cm; l is the length the light travels through the substance in cm, l is generally 1cm; c is the concentration of the substance in moles/L.

If e is known, the concentration of a substance in solution can be determined. An unknown concentration may also be determined by comparing the absorbance of a known (standard) concentration of the substance with the absorbance of the unknown concentration.

Absorbance can be measured using an instrument as advanced as a double beam scanning UV/VIS absorption spectrophotometer or as simple as a colorimeter. The principle of absorbance is also utilized in detectors for other instrumental techniques such as HPLC determination.