This lecture delves into the interpretation of the Fourier transform, discussing the calculation of Fourier transforms, the reciprocity theorem, and its direct verification through residue calculations. The Fourier transform is explored in the context of signals, such as sound waves, with a focus on modifying signals by attenuating or enhancing specific frequencies. The lecture also covers the application of Fourier transforms in optics, particularly in diffraction through lenses. Through detailed mathematical analysis and examples, the instructor demonstrates the calculation of Fourier transforms and the reciprocity theorem, showcasing their relevance in various fields.