An electron (mass $m$ ) with an initial velocity $\vec{v}=v_0 \hat{i}\left(v_0>0\right)$ is moving in an electric field $\vec{E}=-E_0 \hat{i}\left(E_0>0\right)$ where $E_0$ is constant. If at $t=0$ de Broglie wavelength is $\lambda_0=\frac{h}{m v_0}$, then its de Broglie wavelength after time $t$ is given by [MP/Toug\#]