Let λα,λβ and λα′ denote the wavelengths of the X-rays of the Kα,Kβ and Lα in the characteristic X-rays for a metal
λα>λα′>λβ
λα′>λβ>λα
1λβ=1λα+1λα′
1λα+1λβ=1λα′
According to the energy diagram of X-ray spectra
∵ ΔE=hcλ⇒λ∝1ΔE So, Eβ=Eα+Eα′ or 1λβ=1λα+1λα′