This lecture covers the principles of UV-Visible spectrophotometry, focusing on electron transitions, chromophores, and auxochromes. It explains the analysis of substances based on absorption of electromagnetic radiation, with examples of molecules and their corresponding absorption wavelengths. The lecture also discusses the effects of solvents on absorption, the application of UV-Visible spectrophotometry in studying biomolecules, and the importance of pH in protein analysis. Various methods for protein quantification and the detection of external factors on biomolecules are explored, along with examples of absorption spectra for proteins and oligonucleotides.