When electromagnetic radiation interacts with matter, absorption occurs, if the frequency of radiation corresponds precisely to the energy required to raise the system from a particular energy level to a higher allowed energy level. There are two fundamental laws which govern the fraction of incident radiation absorbed on passing through a given sample medium. The first law was stated by Lambert in 1768 and called Lambert’s law. The second law was stated by Beer in 1852 and called Beer’s Law. The combined form of these two laws is called Beer-Lambert law.