A stream of electrons accelerated by a potential difference V falls on the surface of a metal

Question:

A stream of electrons accelerated by a potential difference V falls on the surface of a metal whose inner potential is Vi = 15 V. Find:
(a) The refractive index of the metal for the electrons accelerated by a potential difference V = 150 V;
(b) The values of the ratio V/Vi at which the refractive index differs from unity by not more than η = 1.0%.