The plot of square root of frequency of X-ray emitted against atomic emitted  led to the suggestion of which law /rule?

Moseley gave a modem periodic law.'the square root of the frequency of a line of X-ray spectrum is related to the atomic number Z of the target material as, = a(Z -b)where, a =proportionality constantZ =atomic numberb = another constant(It has the same value for all the lines of X-ray spectrum)