Figure 3
From: Huygens' Principle geometric derivation and elimination of the wake and backward wave
The geometry of an infinite planar impulsive source moving toward P with speed v. (a speed away from P would be given by − v.) R1 is the radius of the circle created by the intersection at time t of a sphere of radius ct with the plane. R2 is the radius of the circle created by the intersection at time t + ε of a sphere of radius c[t + ε] with the plane.
