# Translations:Dictionary:Kirchhoff’s equation/1/en

**1**. An integral form of the wave equation expressing the wave function at the point *P* as the sum of wave contributions from the surroundings. Wave contributions have to allow for the traveltime from the sources to *P*, that is, what the source does at an earlier time affects *P* at time *t*, where *r* is the distance from the source to *P* and *V* is the velocity. The earlier time is called **retarded time**. is expressed as an integral over the volume surrounding *P* (to accommodate sources within the volume) plus an integral over the surface surrounding the volume (to accommodate sources from outside). In source-free space in terms of the values of and its derivative on a surrounding surface *S* at the preceding time :