Frequencies of ${\mathrm{K}}_{\mathrm{\alpha }}$ X-rays of different materials are measured. Which one of the graphs in the figure may represent the relation between the frequency $v$ and the atomic number $Z$.

  

The figure shows the intensity-wavelength relations of $X‐$rays coming from two different Coolidge tubes. The solid curve represents the relation for the tube $A$ in which the potential difference between the target and the filament is $V_{\mathrm{A}}$ and the atomic number of the target material is $Z_{\mathrm{A}}$. These quantities are $V_{\mathrm{B}}$ and $Z_{\mathrm{B}}$ for the other tube. Then,

For harder X-ray,