$X$-rays incident on a material:

The wavelength of the ${K_\alpha}$ line for an element of atomic number $29$ is $\lambda$. Then the wavelength of the ${K_\alpha}$ line for an element of atomic number $15$ is (Take Moseley's constant $b = 1$ for both elements).

For the structural analysis of crystals, $X-$ rays are used because

The variation of wavelength $\lambda$ of the $K_{\alpha}$ line with atomic number $Z$ of the target is shown by the following curve of

According to Moseley's law,the frequency of a spectral line in the $X$-ray spectrum varies as:

If a voltage applied to an $X$-ray tube is increased to $1.5$ times, the minimum wavelength $(\lambda_{\min})$ of an $X$-ray continuous spectrum shifts by $\Delta \lambda = 26 \, pm$. The initial voltage applied to the tube is: