Question

why is it necessary that field lines from a point charge is the vicinity of a conductor must be normal to the conductor at every point.

Answer 1

Because if they are not normal to the conductor then it means that there is a component of them in the horizontal and vertical direction which is not true in this case..
Hope it helps..

Answer 2

Because the surface of a conductor [in Fig 3.23.2] is necessarily a surface of constant potential, the electric field, which is −∇φ−∇φ, must be perpendicular to the surface at every point on the surface
I have omitted the picture because it is not relevant.

What is the reasoning?

I understand that the potential, φφ, is a continuous function, and since E=0E=0 inside the conductor and since E=−∇φE=−∇φ I get that φ=0φ=0 inside and on the surface (from continuity) of the conductor.

However, I don't understand the reason the book gives for explaining why the field is perpendicular to every point on the surface.....

hope it will help...