When electromagnetic radiation reacts with matter the effect depends on the frequency of the radiation.Infra red radiation causes bonds to vibrate, radio frequency waves cause nuclear magnetic resonance and visible light causes electrons to move to an excited state and u.v. radiation breaks bonds.We can determine the concentration of a solution by measuring how much light is absorbed. White light is shone through a filter. The filter only lets through the colour of light that is absorbed by the sample. We then measure how much of that light is absorbed by measuring how much is transmitted. The more concentrated the solution, the more light is absorbed.