Diffraction: Coherence in Optics presents a detailed account of the course on Fraunhofer diffraction phenomena, studied at the Faculty of Science in Paris. The publication first elaborates on Huygens' principle and diffraction phenomena for a monochromatic point source and diffraction by an aperture of simple form. Discussions focus on diffraction at infinity and at a finite distance, simplified expressions for the field, calculation of the path difference, diffraction by a rectangular aperture, narrow slit, and circular aperture, and distribution of luminous flux in the airy spot. The book then examines Fourier transformation and diffraction by several apertures. The manuscript takes a look at extended luminous sources and objects and diffraction phenomena in perfect optical instruments. Topics include diffraction gratings, phase contrast, apodization, filtering spatial frequencies, relation between the length of wave trains and the spectral width of the emitted radiations, successive wave trains emitted by an atom, and vibrations with different frequencies from a single atom. The manuscript also reviews diffraction phenomena in real optical instruments, including diffraction in the case of a spherical wave and diffraction spot in the presence of aberrations. The publication is a dependable reference for readers interested in diffraction phenomena.