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

1. An integral form of the wave equation expressing the wave function ${\displaystyle \psi _{p}}$ 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 ${\displaystyle \tau =(t-r/V)}$ affects P at time t, where r is the distance from the source to P and V is the velocity. The earlier time ${\displaystyle \tau }$ is called retarded time. ${\displaystyle \psi _{p}}$ 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 ${\displaystyle \psi }$ and its derivative on a surrounding surface S at the preceding time ${\displaystyle (t-r/V)}$: