Kinetic energy gained by a charged particle accelerated by a potential V is qV
KE=qV
Using relation between momentum and kinetic energy
⇒2mp2=qV⇒p=2mqV
We know that momentum of charged particle is p=λh.
Thus, λ=2mqVh
Now, λdλp=mpVpmdVd
⇒21=Vp2Vd⇒VdVp=4