Kirchhoff's diffraction formula
Kirchhoff's diffraction formula (also called Fresnel–Kirchhoff diffraction formula) approximates light intensity and phase in optical diffraction: light fields in the boundary regions of shadows. The approximation can be used to model light propagation in a wide range of configurations, either analytically or using numerical modelling.
Source: Wikipedia — Kirchhoff's diffraction formula (CC BY-SA 4.0)