When a colored solution is irradiated with white light, the solution will selectively absorb incident light of some wavelengths but not others. By exposing a colored solution to various wavelengths via spectrophotometry and recording the responses, one can calculate percent transmittance.

If a particular wavelength is not absorbed, the intensity of the beam directed at the solution will match the intensity of the beam transmitted by the solution. If some light is absorbed, the intensity of the beam transmitted by the solution will be less than that of the incoming beam.

The ratio of the intensity of light transmitted by the solution to the intensity of the light absorbed by the solution is the percent transmittance of the solution.

%T = I(transmitted) / I(absorbed) x 100%

The wavelength at which percent transmittance is lowest is the wavelength to which the solution is most sensitive. This wavelength, used in spectrophotometric analysis, is the analytical wavelength.

Once the percent transmittance is found, one can calculate the absorbance of the solution using the equation

A = 2.000 - log(%T)

Once absorbance is found, you can use Beer's Law to calculate molar concentration. Using a Beer's Law plot, you can find the molar concentration of a solution if you know the percent transmittance (and hence the absorbance).