| . . . . . . | I0The most common statement of the law is:
> |. . c,α . . .|> I1 | . . . . . . | <- - - l - - ->
In essence, the law states that there is an exponential dependence between the transmission of light through a substance and the concentration of the substance, and also between the transmission and the length of material that the light travels through. Thus if l and α are known, the concentration of a substance can be deduced from the amount of light transmitted by it.
The units of c and α depend on the way that the concentration of the absorber is being expressed. If the material is a liquid, it is usual to express the absorber concentration c as a mole fraction i.e. a dimensionless fraction. The units of α are thus reciprocal length (e.g. cm-1). In the case of a gas, c may be expressed as a density (units of reciprocal length cubed, e.g. cm-3), in which case α is an absorption cross-section and has units of length squared (e.g. cm2).
The value of the absoption coefficient α varies between different absorping materials and also with wavelength for a particular material. It is usually determined by experiment.
The law tends to break down at very high concentrations, especially if the material is highly scattering. If the light is especially intense, nonlinear optical processes can also cause variances.
The law's link between concentration and light absorption is the basis behind the use of spectroscopy to identify substances.
See also: absorption.