Abstract
It is the purpose of this paper to study the reflection of solutions of Helmholtz equation with Neumann boundary data. In detail let u be a solution of Helmholtz equation in the exterior of a ball in R$^3$ with exterior Neumann data ∂(sub)νu = 0 on the boundary of the ball. We prove that u can be extended to R$^3$ except the center of the ball. As a corollary, we prove that a sound hard ball can be identified by the scattering amplitude corresponding to a single incident direction and as single frequency.