An electron (mass m) with initial velocity ν→=ν0i^+ν0j^  is in an electric field  E→=−E0k^. If  λ0 is initial de-Broglie wavelength of electron, its de-Broglie wavelength at time t is given by :

Initially momentum =p=m2V0=hλ0​⇒hm=2V0λ0     .........1 Acceleration of electron = a→=eE0mk^​Using equation of kinematicsv→=u→+a→t​⇒v→ = v0i^+v0j^+eE0tmk^Magnitude of velocity =  v→=v02+v02+eE0tm2     So,  wavelength = λ=hmv=hm2v02+e2E02t2m2 ⇒λ=2 v0λ02v0×1+e2E022m2v02t2=λ01+e2E022m2v02t2