

Let us consider a conductor with a cavity or a hole. Electric field inside the cavity is zero.
Let E is the electric field just outside the conductor, q is the electric charge,
is the charge density, and
is the permittivity of free space.
Charge 
According to Gauss’s law,



Therefore, the electric field just outside the conductor is
. This field is a superposition of field due to the cavity
and the field due to the rest of the charged conductor
. These fields are equal and opposite inside the conductor, and equal in magnitude and direction outside the conductor.



Therefore, the field due to the rest of the conductor is
.
Hence, proved.
