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 \(\eta=1.0 \%\).