An electron of mass m with an initial velocity $\overrightarrow{V} = V_0 \hat{i} (V_0 > 0) $ enters an electric field $\overrightarrow{E} = -E_0 \hat{i} (E_0 = constant > 0)$ at $t = 0$. If $\lambda_0$ is its de-Broglie wavelength initially, then its de-Broglie wavelength at time t is :