Use Moseley's law with b = 1 to find the frequency of the `K_alpha` X-ray of `La(Z = 57)` if the frequency of the `K_alpha` X-ray of `Cu(Z = 29)` is known to be `1*88 xx 10^(18)` Hz.

Explanation

Using the equation. `sqrt(f)=alpha (Z-b)" " ("for K-series",b=1)`
`rArr" " (f_(La))/(f_(Cu))=((Z_(La-1))/(Z_(Cu-1)))or " " f_(L_(a))=f_(Cu)((Z_(La)-1)/(Z_(Cu)-1))^(2)`
`" " =1.88xx10^(18)((57-1)/(29-1))^(2)=7.52xx10^(18)Hz`