Also known as Beer-Lambert Law. This is a very important yet simple relationaship between how much light is absorbed and the concentration of the absorbing species in the sample.
Transmittance: T = P / Po
Absorbance: A = -log T = log (Po/P)
Beer's Law: A = e b c
A = absorbance, e = molar absorbtivity (M-1 cm-1),
b = path length (cm), c = concentration (M)
Molar absorbtivity is a characteristic of a particular substance which determines how much light will be absorbed at any given wavelength.