Use Moseley's law with b=1 to find the frequency of the `K_(alpha)` X-rays of La (Z=57) , if the frequency of the `K_(alpha)` X-rays of Cu (Z=29) is know to be `1.88xx10^(18)Hz.`

Explanation

Given `b=1 `
For La,`a=(57)`
` sqrtf=a(Z-b) `
` rArr sqrtf=a(57-1) `
` =bxx56……(1) `
` for Cu(29) `
` sqrt1.88xx10^18=a(29-1)=28a…..(2) Dividing (1) and (2) `
` sqrt(f/(1.88xx10^18))=(axx56)/(axx28) =2 `
` rArr f=1.88xx10^18(2)^2 `
` =4xx1.88xx10^18 `
` 7.52xx10^18Hz.`