In absorption spectrometry the extent to which the intensity of the transmitted radiation decreases will depend on the path length through the sample and on the concentration of the absorbing species. In addition to these two quantities, absorptivity (formerly called the extinction coefficient)is another quantity that depends on the identity of the absorbing substance. Absorptivity is a useful quantity. It gives us an idea about the transition probabilities in the molecules. If its value is known, then concentration of an unknown solution of the absorbing species can be determined from its absorbance measurements.