UV/VIS spectroscopy is the experimental method used for determination of the concentration of a material in the sample by measuring the amount of light absorbed by a sample. The method is based on the Lambert-Beer's law that provides the function ratio between experimentally determined physical quantity, absorbance (A) and the physical quantity determined by calculation, concentration (c). Concentration can be determined graphically from the calibration curve of dependence of absorbance on concentration. The interaction between photons and particles that absorb electromagnetic radiation is the reduction of beam intensity from \( \mathit{I_{0}} \) to \( \mathit{I} \). The law can be represented as: \( \mathit{A}= log \left ( \ \mathit{I_{0}}/\mathit{I} \right )= \mathit{\varepsilon }bc \) where A is the absorbance at a given wavelength of light, \( \mathit{\varepsilon }\) is the molar absorption (extinction) coefficient expressed in L \( mol^{-1} cm^{-1} \), that is characteristic for each molecule and dependent on the wavelength of light, b is the length of the light path through the sample expressed in cm and c the concentration of the substance in the solution expressed in mol \( L^{-1} \). This is the limiting law because it is only valid for low concentrations and for monochromatic light sources. Lambert-Beer's law and UV/Vis spectrophotometry are very useful because they provide quantification and determination of DNA purity of biological samples, chemical reaction monitoring, substance identification, determination of different substances and the like.