At the interface between one linear dielectric and another, the electric field lines ben...

At the interface between one linear dielectric and another, the electric field lines bend (see Fig. 4.34). Show that

assuming there is no free charge at the boundary. [Comment: Eq. 4.68 is reminiscent of Snell’s law in

optics.Would a convex “lens” of dielectric material tend to “focus,” or “defocus,” the electric field?]