Given X-ray spectrum is for a Coolidge tube having accelerating potential V. If accelerating potential is decreased to V/4, then Δλ=(λ−λc) becomes four times with changes in anode element atomic no. from (neglect screening effect).
Z/2
Δλ1=(λ1−λ1c)=hcZ12(Ek−EL)−hceV1=4(hcZ2(Ek−EL)−hceV)⇒Z1=Z/2