But this result cannot be correct, since It is well known that a charge Q in a medium of dielectric constant ϵ ϵ 3 at a distance D from the plane surface of a second medium of dielectric constant ϵ ϵ 2 experiences a force as if there were an 'image' charge of strength Q ′ = − − Q ( ϵ ϵ 2 − − ϵ ϵ 3 ) / ( ϵ ϵ 2 + ϵ ϵ 3 ) at distance D on the other side of the boundary. The force between the real and image charges must then be