Problem 6.63

A stream of electrons accelerated by a potential difference $V$ falls on the surface of a metal whose inner potential is $V_{i}=15 \mathrm{~V}$. Find: (a) the refractive index of the metal for the electrons accelerated by a potential difference $V=150 \mathrm{~V}$; (b) the values of the ratio $V / V_{i}$ at which the refractive index differs from unity by not more than $\eta=1.0 \%$.