Absorbance is a chemical property specifying how much of a wavelength of light is absorbed by a material in solution. It is defined by the following relation:

A=log(1/T)

where T is the transmittance of the radiation passing through a sample of material. A material that absorbs 90% of the light passing through it would be said to have a transmittance of 10%. It would have an absorbance of 1. The absorbance of a material at a specific wavelength is related to its concentration in a solution through Beer's Law.

The absorbance of a substance varies with the wavelength of the radiation. Substances can only absorb radiation if they are "allowed" to by quantum mechanical laws. Excitation of vibrational, rotational, and electronic modes of molecules can only occur at specific frequencies. Spectroscopic methods such as FTIR or UV/Vis spectroscopy use this fact as a method of identifying unknown materials through their absorbance fingerprint.

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