Introduction to Beer's Law

Substances that are coloured absorb ultraviolet light and appear as the complement of the colour that they absorb. Complementary colours are opposite on the colour wheel.
 The diagram below shows that if a sample absorbs light of a particular wavelength, the intensity of the light is diminished after light has passed through the sample. 

The ratio of the intensity of the incident light (I_o) to the intensity of the light leaving the sample (I) is referred to as the transmittance T. The transmittance depends on the concentration of the absorbing substance, the distance that the light travels through the cell (l = ell = length) and the molar absorptivity of the sample (ε)at the wavelength at which the measurement is made.

T =	I	= 10–εcl
	Io

It is preferable to use absorbance (A) as defined below because absorbance is directly related to the concentration, cell path length and molar absorptivity of the compound. The relationship between these is known as Beer's Law.

A = –log	I		A = εcl Beer's Law
	Io

 
Note that A is dimensionless and that the unit of ε depends on the unit of concentration and distance (I) used in the experiment.

An ultraviolet spectrum is a plot of absorbance versus wavelength for a sample of the absorbing substance.  These spectra have broad peaks.  When analyzing samples using Beer's Law it is common to choose a wavelength at which the absorption is the maximum.