**SOLUTION : **$\quad E=m c^{2} ; E=h v$

$\begin{aligned} \therefore \quad & m c^{2}=h v \quad \therefore \quad v=\frac{m c^{2}}{h}=\frac{m c \times c}{h} \\=& \frac{1.1 \times 10^{-23} \times 3 \times 10^{8}}{6.626 \times 10^{-34}}=5 \times 10^{18} Hz \end{aligned}$