Calculate the wavelength of the $k_{\alpha}$ line for $z=31$ when $a=5 \times 10^{7}\;Hz^{1/2}$ for a characteristic X-ray spectrum.

Question

\[\begin {array} {1 1} (a)\;1.33\;A & \quad (b)\;1.33\;nm \\ (c)\;133 \times 10^{-10}\;m & \quad (d)\;133 \;nm \end {array}\]