Show that (vector k vector r, t) may represent a plane wave where vector k is normal

Show that Ψ(vector k• vector r, t) may represent a plane wave where vector k is normal to the wavefront. [Let vector r₁ and vector r₂ be position vectors drawn to any two points on the plane and show that ψ( vector r₁, t) = ψ( vector r₂, t).]