(b) For incident electron $\frac{1}{2}mv^{2} = eV$ or$\ p^{2} = 2meV$
$\therefore$ de-Broglie wavelength $\lambda_{1} = \frac{h}{p} = \frac{h}{\sqrt{2meV}}$
Shortest $X$-ray wavelength $\lambda_{2} = \frac{hc}{eV}$
$\therefore\frac{\lambda_{1}}{\lambda_{2}} = \frac{1}{c}\sqrt{\left( \frac{V}{2} \right)\left( \frac{e}{m} \right)} = \frac{\sqrt{\frac{10^{4}}{2} \times 1.8 \times 10^{11}}}{3 \times 10^{8}} = 0.1$