Henry Moseley studied characteristic X -ray spectra of elements....

Moseley's observation on characteristic $X$-ray spectra of elements relates frequency $( v )$ of $X$-rays emitted to the atomic number by the following relation:
$\sqrt{v} = a ( Z - b )$
The plot of $\sqrt{v}$ vs $Z$ is linear.

Based on this observation, the periodic law defined as the physical and chemical properties of the elements are periodic functions of their atomic numbers.