#OTD in 1818.
French physicist Augustin Fresnel signs his preliminary "Note on the Theory of Diffraction" (deposited on the following day). The document ends with what we now call the Fresnel integrals.
The Fresnel integrals have various applications in optics, such as in the calculation of the diffraction pattern produced by a single slit or a circular aperture, as well as in the study of the propagation of light through various optical systems.