λ = h/mv

v = at = eEt/m

λ = h/eEt

Hii

Since electric field is applied on the electron so obviously it will get accelerated..

and its acceleration will have a numerical value equal to eE/m {eE is foirce due to EF and m is mass}

Now since initial velocity is zero..

so at time t

v = (eE/m).t

Now debroglie's wavelength = h/mv

lambda = h/eEt

d(lambda)/dt = -h/eEt^2

so on increasing time...debroglies wavelength decreses..{negative sign shows decreament}